style: Avoid optional parameter syntax

Supporting this syntax uncorks a whole bottle of worms and requires participation in more of the
JavaScript ecosystem and is not worthwhile in my opinion at this time.

Fixes #873

Author: tmcw

docs/NODE_API.md

@@ -105,7 +105,7 @@ Formats documentation as HTML.
 **Parameters**
 
 -   `comments` **[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)<[Comment](https://developer.mozilla.org/en-US/docs/Web/API/Comment/Comment)>** parsed comments
--   `config` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output (optional, default `{}`)
+-   `config` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output
     -   `config.theme` **[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** Name of a module used for an HTML theme. (optional, default `'default_theme'`)
 
 **Examples**
@@ -132,7 +132,7 @@ Formats documentation as
 **Parameters**
 
 -   `comments` **[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)<[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)>** parsed comments
--   `args` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output (optional, default `{}`)
+-   `args` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output
 
 **Examples**
 

src/output/html.js

@@ -23,7 +23,10 @@ var mergeConfig = require('../merge_config');
  *     streamArray(output).pipe(vfs.dest('./output-directory'));
  *   });
  */
-function html(comments: Array<Comment>, config: Object = {}) {
+function html(comments: Array<Comment>, config?: Object) {
+  if (!config) {
+    config = {};
+  }
   return mergeConfig(config).then((config: DocumentationConfig) => {
     var themePath = '../default_theme/';
     if (config.theme) {

src/output/markdown.js

@@ -23,10 +23,10 @@ var remark = require('remark'),
  *     fs.writeFileSync('./output.md', output);
  *   });
  */
-function markdown(
-  comments: Array<Comment>,
-  args: Object = {}
-): Promise<string> {
+function markdown(comments: Array<Comment>, args?: Object): Promise<string> {
+  if (!args) {
+    args = {};
+  }
   return markdownAST(comments, args).then(ast => remark().stringify(ast));
 }