Added support for "*.sass" Sass files in addition to "*.scss"

Hello,

Sass has both a `*.scss` and a `*.sass` syntax variant, with AssertPack so far only supporting the former. As the `sass` preprocessor handles both, support for the `*.sass` variant can be added trivially, which I have done.  It would be nice if this could be included in the mainline.

Cheers,

Marco