Add ability to extract comments from the grammar

All comments stored in the `comments` property of the `grammar` node.
Comments extracted only if the `extractComments` options set to `true` when you generate parser.
This property is object with mapping start offset of comment to comment object, that looks like:

```js
{
  text: 'text in the comment, just after // or /* and before */',
  multiline: true|false,// true for /**/ comments, false for // comments
  location: location()
}
```

README.md

@@ -188,6 +188,25 @@ object to `peg.generate`. The following options are supported:
   * `plugins` — plugins to use
   * `trace` — makes the parser trace its progress (default: `false`)
 
+Also you can supply boolean option `extractComments` (default: `false`).
+When set to `true`, parser will be collect all comments in the grammar to the object
+in the `comments` property in the `grammar` AST node. This key contains mapping from
+offset position of start location of the comment (i.e. `//` or `/*`) to comment object
+itself. Comment object has following structure:
+
+  ```js
+  {
+    text: 'all text between /* or */, or // and end of line',
+    multiline: true|false,
+    location: location()
+  }
+  ```
+
+When set to `false`, `comments` will be set to `null`.
+
+This option not impact to the generated parser but only to grammar AST, which
+can be used by plugins.
+
 Using the Parser
 ----------------
 

lib/parser/ast.js

@@ -15,11 +15,12 @@ exports.Node = Node;
 class Grammar extends Node {
 
     // Creates a new AST
-    constructor( initializer, rules, location ) {
+    constructor( initializer, rules, comments, location ) {
 
         super( "grammar", location );
 
         this.initializer = initializer;
+        this.comments = comments;
         this.rules = rules;
 
     }