Passing filename to Stylus API

As recommended by https://github.com/LearnBoost/stylus/blob/master/docs/js.md
Reading config from config.plugins.stylus

Backward compatible and deprecation message for config.stylus
Allowing Stylus linenos option in config
Documenting debug options & config.plugins nesting
DRYer @config handling
Improved markdown coffeescript block
Merge pull request #17 from es128/master

Configurable linenos option for debug comments