support filters

jade mentions [two filters](http://jade-lang.com/reference/filters/) in the docs: `markdown` and `coffee`.

this adds `coffee-script` and `marked`, which will allow those two to be run successfully.

I am sure there are others, but having these would be very nice for my workflow for quickly hacking up examples for code that I am working on.

if you don't want these to be included in the base install (understandable) is there a way to install/reload packages from the options screen?