Adds a Stylus-like API for browserbuild

[mixu]

I wanted to use browserbuild directly from node, so I implemented https://github.com/LearnBoost/browserbuild/issues/14

Supported:

browserbuild(paths, options)
  .set(key, val) // change options
  .include(paths) // add more paths
  .render(fn); // callback(err, txt)

browserbuild.render(paths, options, fn)

[rauchg]

This is beautiful @mixu. I'm almost tearing up

[mixu]

:+1: let's do lunch sometime!