Pub Transformer for Stylus

I hadn't heard of stylus before, but because you've set it up to take input from stdin it scratched an itch of mine to be able to use a CSS preprocessor in HTML files.  So I've written a Barback transformer for your Stylus implementation so it can be used by pub build, pub serve, etc to compile .styl files and find Stylus in HTML files and process that too.