Add basic support for feature queries

This PR implements basic support for `@supports` feature queries. There are almost certainly cases that aren't yet covered but it gets the basic job done.

Fixes https://github.com/sass/libsass/issues/261. Specs added https://github.com/sass/sass-spec/commit/e73cbf3e8580a2e17c1ec82cf2529ad759ce2200.