Add stylesheet detection for .scss extension

I really like this package, but I'm using Sass, so I spent some time trying to add support for it.

Just configuring filters with the extension in `config.php` wasn't enough, because I just couldn't get my `.css.scss` files into `application.css` (At this point, the documentation in README is just plain wrong. Maybe you are already thinking about this in Issue #8, but I thought it is worth pointing out anyway).

Turns out `SprocketsRepository` needs to know about the `.scss` extension to work.

Please add this to support those, who want to use Sass with the asset-pipeline.