Support for pepending requirejs data-main attribute

Suppose html like this:

```html
<script data-main="scripts/main" src="bower_components/requirejs/require.js"></script>
```

After grunt-cdn:

```html
<script data-main="scripts/main" src="http://cdn/bower_components/requirejs/require.js"></script>
```

With this merge, scripts in the `data-main` will also get cdned:

```html
<script data-main="http://cdn/scripts/main" src="http://cdn/bower_components/requirejs/require.js"></script>
```