Add support for reading report arguments from .netconfig

The http://dotnetconfig.org/ format allows unifying configuration with
other .NET core tools, making it easier to maintain multiple tool settings
in a centralized fashion.

By leveraging the format, ReportGenerator gains:

* Simplified command line args in CI scripts (just run without parameters
  most of the time)
* Support for hierarchical configurations
* Standard way of updating and managing those configurations, via the
  `dotnet config` tool, such as `dotnet config --add reportgenerator.reporttype Html`

The configuration is shared uniformly between the CLI and MSBuild task, for
consistency. Updated the documentation too to showcase the new functionality.

Author: kzu

README.md

@@ -103,7 +103,52 @@ Examples:
    "-reports:coverage.xml" "-targetdir:C:\report" "-assemblyfilters:+Included;-Excluded.*"
 ```
 
-**MSBuild**  
+#### .netconfig support
+
+All the above parameters can also be persisted in a [.netconfig](https://dotnetconfig.org) file, under a `[ReportGenerator]` 
+section. Examples:
+
+```
+[ReportGenerator]
+	reports = coverage.xml
+	targetdir = "C:\report"
+	reporttypes = Latex;HtmlSummary
+	assemblyfilters = +Test;-Test
+	classfilters = +Test2;-Test2
+```
+
+All the plural options can also be specified as multiple singular entries, like:
+
+```
+[ReportGenerator]
+	report = coverage1.xml
+	report = coverage2.xml
+	reporttype = Latex
+	reporttype = HtmlSummary
+	assemblyfilter = +Test
+	assemblyfilter = -Test
+	classfilter = +Test2
+	classfilter = -Test2
+	filefilter = +cs
+	filefilter = -vb
+	sourcedir = src
+	sourcedir = test
+```
+
+Adding/removing values is trivial using the [dotnet-config](https://dotnetconfig.org) CLI:
+
+```
+# set a single-valued variable
+dotnet config reportgenerator.reporttypes Latex;HtmlSummary
+# add to multi-valued variable
+dotnet config --add reportgenerator.report coverage3.xml
+# clear all multi-valued entries for a variable
+dotnet config --unset-all reportgenerator.assemblyfilter
+```
+
+Of course it's equally trivial to just edit the `.netconfig` file by hand.
+
+#### MSBuild
 A MSBuild task also exists:
 
 ```xml
@@ -116,11 +161,41 @@ A MSBuild task also exists:
     <ItemGroup>
       <CoverageFiles Include="OpenCover.xml" />
     </ItemGroup>
-    <ReportGenerator ReportFiles="@(CoverageFiles)" TargetDirectory="report" ReportTypes="Html;Latex" HistoryDirectory="history" Plugins="CustomReports.dll" AssemblyFilters="+Include;-Excluded" VerbosityLevel="Verbose" />
+    <ReportGenerator ProjectDirectory="$(MSBuildProjectDirectory)" ReportFiles="@(CoverageFiles)" TargetDirectory="report" ReportTypes="Html;Latex" HistoryDirectory="history" Plugins="CustomReports.dll" AssemblyFilters="+Include;-Excluded" VerbosityLevel="Verbose" />
   </Target>
 </Project>
 ```
 
+The MSBuild task parameters can be complemented with the `.netconfig`, if used. That means that parameters 
+can be omitted if they are provided via `.netconfig`, which is useful when reusing fixed settings across 
+multiple projects in a solution, where the MSBuild task is only provided the dynamic values for the current 
+project:
+
+Given the following `.netconfig`:
+
+```
+[ReportGenerator]
+  reporttypes = Html;Latex
+  targetdirectory = report
+  historydirectory = history
+  assemblyfilters = +Include;-Excluded
+  verbosityLevel = Verbose
+```
+
+The above target could be simplified as:
+
+```xml
+  <Target Name="Coverage">
+    <ItemGroup>
+      <CoverageFiles Include="OpenCover.xml" />
+    </ItemGroup>
+    <ReportGenerator ProjectDirectory="$(MSBuildProjectDirectory)"
+                     ReportFiles="@(CoverageFiles)" 
+                     Plugins="CustomReports.dll" />
+  </Target>
+```
+
+
 ## Supported input and output file formats
 *ReportGenerator* supports several input and output formats.  
 The wiki explains the different [output formats](https://github.com/danielpalme/ReportGenerator/wiki/Output-formats) or you can download [sample reports](https://danielpalme.github.io/ReportGenerator/resources/SampleReports.zip) of all formats.  

src/ReportGenerator.Core.Test/ReportConfigurationBuilderTest.cs

@@ -1,5 +1,7 @@
+using System;
 using System.IO;
 using System.Linq;
+using DotNetConfig;
 using Palmmedia.ReportGenerator.Core.Logging;
 using Xunit;
 
@@ -10,17 +12,21 @@ namespace Palmmedia.ReportGenerator.Core.Test
     /// to contain all ReportConfigurationBuilder Unit Tests
     /// </summary>
     [Collection("FileManager")]
-    public class ReportConfigurationBuilderTest
+    public class ReportConfigurationBuilderTest : IDisposable
     {
         private static readonly string ReportPath = Path.Combine(FileManager.GetCSharpReportDirectory(), "OpenCover.xml");
 
         private ReportConfigurationBuilder reportConfigurationBuilder;
+        private string currentDir;
 
         public ReportConfigurationBuilderTest()
         {
             this.reportConfigurationBuilder = new ReportConfigurationBuilder();
+            this.currentDir = Directory.GetCurrentDirectory();
         }
 
+        public void Dispose() => Directory.SetCurrentDirectory(this.currentDir);
+
         [Fact]
         public void InitWithNamedArguments_OldFilters_AllPropertiesApplied()
         {
@@ -70,5 +76,92 @@ public void InitWithNamedArguments_NewFilters_AllPropertiesApplied()
             Assert.NotNull(configuration.AssemblyFilters);
             Assert.NotNull(configuration.ClassFilters);
         }
+
+
+        [Fact]
+        public void ConfigProvidesMissingArguments()
+        {
+            var dir = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
+            Directory.CreateDirectory(dir);
+            var config = Config.Build(dir).GetSection(ReportConfigurationBuilder.SectionName);
+
+            config.SetString("reports", ReportPath);
+            config.SetString("targetdir", "C:\\temp");
+            config.SetString("reporttype", "Latex");
+            config.SetString("assemblyfilters", "+Test;-Test");
+            config.SetString("classfilters", "+Test2;-Test2");
+            config.SetString("verbosity", VerbosityLevel.Info.ToString());
+
+            Directory.SetCurrentDirectory(dir);
+
+            string[] namedArguments = new string[0];
+
+            var configuration = this.reportConfigurationBuilder.Create(namedArguments);
+
+            Assert.True(configuration.ReportFiles.Contains(ReportPath), "ReportPath does not exist in ReportFiles.");
+            Assert.Equal("C:\\temp", configuration.TargetDirectory);
+            Assert.True(configuration.ReportTypes.Contains("Latex"), "Wrong report type applied.");
+            Assert.True(configuration.AssemblyFilters.Contains("+Test"), "AssemblyFilters does not exist in ReportFiles.");
+            Assert.True(configuration.ClassFilters.Contains("+Test2"), "ClassFilters does not exist in ReportFiles.");
+            Assert.True(configuration.AssemblyFilters.Contains("-Test"), "AssemblyFilters does not exist in ReportFiles.");
+            Assert.True(configuration.ClassFilters.Contains("-Test2"), "ClassFilters does not exist in ReportFiles.");
+            Assert.NotNull(configuration.ReportFiles);
+            Assert.NotNull(configuration.AssemblyFilters);
+            Assert.NotNull(configuration.ClassFilters);
+        }
+
+        [Fact]
+        public void ConfigProvidesMultiValuedSettings()
+        {
+            var dir = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
+            Directory.CreateDirectory(dir);
+            var config = Config.Build(dir).GetSection(ReportConfigurationBuilder.SectionName);
+
+            config.SetString("reports", ReportPath);
+
+            config.AddString("reporttype", "Latex");
+            config.AddString("reporttype", "Html");
+
+            config.AddString("assemblyfilter", "+Test");
+            config.AddString("assemblyfilter", "-Test");
+
+            config.AddString("classfilter", "+Test2");
+            config.AddString("classfilter", "-Test2");
+
+            config.AddString("filefilter", "+cs");
+            config.AddString("filefilter", "-vb");
+
+            config.AddString("sourcedir", "src");
+            config.AddString("sourcedir", "test");
+
+            config.AddString("plugin", "xunit");
+            config.AddString("plugin", "moq");
+
+            Directory.SetCurrentDirectory(dir);
+
+            string[] namedArguments = new string[0];
+
+            var configuration = this.reportConfigurationBuilder.Create(namedArguments);
+
+            Assert.Contains(ReportPath, configuration.ReportFiles);
+
+            Assert.Contains("Latex", configuration.ReportTypes);
+            Assert.Contains("Html", configuration.ReportTypes);
+
+            Assert.Contains("+Test", configuration.AssemblyFilters);
+            Assert.Contains("-Test", configuration.AssemblyFilters);
+
+            Assert.Contains("+Test2", configuration.ClassFilters);
+            Assert.Contains("-Test2", configuration.ClassFilters);
+
+            Assert.Contains("+cs", configuration.FileFilters);
+            Assert.Contains("-vb", configuration.FileFilters);
+
+            Assert.Contains("src", configuration.SourceDirectories);
+            Assert.Contains("test", configuration.SourceDirectories);
+
+            Assert.Contains("xunit", configuration.Plugins);
+            Assert.Contains("moq", configuration.Plugins);
+        }
     }
 }