add stylus syntax highlighting

 - Stylus on Github: https://github.com/LearnBoost/stylus
 - Stylus: expressive, robust, dynamic css (http://learnboost.github.io/stylus/)
 - I personally prefer id, class, and pseudo to all just be "tag", but i kept the differences for consistency with css/scss highlighters and more customizable styling
 - i required $ for variable names even though stylus doesn't require them because figuring out real variables in stylus is a huge pain with its very minimal syntax, and $ is common (ex: https://github.com/LearnBoost/stylus/issues/1547)
 - also because of the minimal syntax, i tried to be very robust with certain rules like what can go before and after functions/tags

![screen shot 2014-08-23 at 12 48 29 am](https://cloud.githubusercontent.com/assets/1251587/4019965/a5fd1304-2a99-11e4-9cf4-f4714fe68855.png)