Adding comment wrapper to define required srcs

Heya,

Not sure if this is useful.  I had a use case were I was required to exclude a js library from the dist build, so came up with this.

```html
<!-- build:js -->
<script src="js/foo.js"></script>
<script src="js/bar.js"></script>
<script src="js/baz.js"></script>
<!-- endbuild -->
<script src="js/notInBuild.js"></script>
```

If it's of no use, let me know and I'll remove the pull request.

Cheers,
Jozz