Feature/stylecow

Hi there,

Great project!

I've added a filter for the Stylecow css processor (https://github.com/oscarotero/stylecow). Stylecow adds a few new features that the existing filters don't provide (at least from what I can see). The one I was interested in was the automatic generation of css vendor prefixes from non-vendor prefixed (standard) rules.

Note that I'm not affiliated with Stylecow.

Cheers!