Support comments in JavaScript and HTML files

The following changes will enable the extractor to extract "extracted comments" from javascript and html files.

Example,
**JavaScript**
```javascript
angular.module("myApp").controller("helloController", function (gettext) {
   /// this is a comment
    var myString = gettext("Translate This String");
});
```
**HTML**
```html
<p translate-comment="this is a comment" translate>Translate This String</p>