Add `sass` syntax to csscomb.coffee

csscomb supprts the sass syntax for quite a while now: https://twitter.com/csscomb/status/491128538238844929

[Original commit in the csscomb repo](https://github.com/csscomb/csscomb.js/commit/0fca0f8ff2568a48a0a40da73af4498426df808a)