Add support for SCSS files

Should close #35.

Be aware that the file type must be set to `scss`, not `scsscss`, `scss.css` or anything like that. It might be necessary to manually set the file type with a command like `:set ft=scss`.

The test file `example.scss` should look like this:
![preview](https://cloud.githubusercontent.com/assets/189879/2585144/494dafd2-b9e8-11e3-88e0-435930a952e3.png)