Add rule max-attributes-per-line (resolves #47)

docs/rules/max-attributes-per-line.md

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+# Define the number of attributes allows per line (max-attributes-per-line)
+
+Limits the maximum number of attributes/properties per line to improve readability.
+
+
+## Rule Details
+
+This rule aims to enforce a number of attributes per line in templates. It checks all the elements in a template and verifies that the number of attributes per line does not exceed the defined maximum.
+An attribute is considered to be in a new line when there is a line break between two attributes.
+
+There is predefined number of attributes that are acceptable in one-line case, as well as how many attributes are acceptable per line in multi-line case.
+
+The default is one attribute per line for multiline and three attributes per line in singleline.
+
+Examples of **incorrect** code for this rule:
+
+```js
+<component lorem="1" ipsum="2" dolor="3" sit="4">
+</component>
+
+<component
+  lorem="1" ipsum="2"
+  dolor="3"
+  sit="4"
+>
+</component>
+```
+
+Examples of **correct** code for this rule:
+
+```js
+<component lorem="1" ipsum="2" dolor="3">
+</component>
+
+<component
+  lorem="1"
+  ipsum="2"
+  dolor="3"
+  sit="4"
+>
+</component>
+
+```
+
+### Options
+
+```
+{
+  "vue/max-attributes-per-line": ["error", {
+    "firstline": 1,
+    "singleline": 2,
+    "multiline": 1
+  }]
+}
+```
+
+### `firstline`
+For muliline declarations, defines if it accepts an attribute in the first line.
+
+The following patterns is considered a warning:
+```js
+// ["error", { "firstline": 1, "singleline": 2, "multiline": 1}]
+<component
+  foo="John" bar="Smith"
+  foobar={5555555}>
+</component>;
+```
+
+The following pattern is not considered a warning:
+```js
+// ["error", { "firstline": 0, "singleline": 3, "multiline": 1}]
+<component
+  foo="John"
+  bar="Smith"
+  foobar={5555555}
+  >
+</component>;
+```
+
+
+### `singleline`
+Number of maximum attributes per line when the opening tag is in a single line.
+
+The following patterns is considered a warning:
+```js
+// ["error", { "firstline": 1, "singleline": 2, "multiline": 1}]
+<component foo="John" bar="Smith" foobar={5555555}></component>;
+```
+
+The following pattern is not considered a warning:
+```js
+// ["error", { "firstline": 1, "singleline": 3, "multiline": 1}]
+<component foo="John" bar="Smith" foobar={5555555}></component>;
+```
+
+
+### `multiline`
+Number of maximum attributes per line when a tag is in multiple lines.
+
+
+The following patterns is considered a warning:
+```js
+// ["error", { "firstline": 1, "singleline": 2, "multiline": 1}]
+<component foo="John" bar="Smith"
+  foobar={5555555}>
+</component>;
+```
+
+The following pattern is not considered a warning:
+```js
+// ["error", { "firstline": 0, "singleline": 3, "multiline": 1}]
+<component
+  foo="John"
+  bar="Smith"
+  foobar={5555555}
+  >
+</component>;
+```
+
+## When Not To Use It
+
+If you do not want to check the number of attributes declared per line you can disable this rule.
+

lib/rules/max-attributes-per-line.js

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+/**
+ * @fileoverview Define the number of attributes allows per line
+ * @author Filipa Lacerda
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        docs: {
+            description: "Define the number of attributes allows per line",
+            category: "Fill me in",
+            recommended: false
+        },
+        fixable: null,  // or "code" or "whitespace"
+        schema: [
+            // fill in your schema
+        ]
+    },
+
+    create: function(context) {
+
+        // variables should be defined here
+
+        //----------------------------------------------------------------------
+        // Helpers
+        //----------------------------------------------------------------------
+
+        // any helper functions should go here or else delete this section
+
+        //----------------------------------------------------------------------
+        // Public
+        //----------------------------------------------------------------------
+
+        return {
+
+            // give me methods
+
+        };
+    }
+};

tests/lib/rules/max-attributes-per-line.js

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+/**
+ * @fileoverview Define the number of attributes allows per line
+ * @author Filipa Lacerda
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+var rule = require("../../../lib/rules/max-attributes-per-line"),
+
+    RuleTester = require("eslint").RuleTester;
+
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+var ruleTester = new RuleTester();
+ruleTester.run("max-attributes-per-line", rule, {
+
+    valid: [
+
+        // give me some code that won't trigger a warning
+    ],
+
+    invalid: [
+        {
+            code: "",
+            errors: [{
+                message: "Fill me in.",
+                type: "Me too"
+            }]
+        }
+    ]
+});