Support for prepending requirejs data-main attribute; Fix `dest` option

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 commit, 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>
```

The `dest` option is in fact unavailable, mofified content still goes to original file. This commit also fix this.