Adds Stylus support to brunch.

Also includes nib cross-browser mixins.

Add "stylus-brunch": "x.y.z" to package.json of your brunch app.

Pick a plugin version that corresponds to your minor (y) brunch version.

If you want to use git version of plugin, add "stylus-brunch": "git+ssh:https://[email protected]:brunch/stylus-brunch.git".