Allow “*.css.sass” or “*.css.scss” as file extensions.

Hi there,

first of all, thanks for guard-sass.

I just wrote a little importer like in “sass-rails” (https://github.com/rails/sass-rails/blob/master/lib/sass/rails/importer.rb) which overrides the “extensions” method, so you can have “*.css.sass” or “*.css.scss” as file extensions without having to write “@import "foo.css.scss"”.

Fixes / improves #17.