Retain CoffeeScript output #56

A nice enhancement for the sbt-js plugin would be the ability to retain the JS output by the CoffeeScript compiler. Initially I would expect this to be possible given the option to pretty print the javascript, but after configuring the my project to use the plugin I only see the closure-compiled output. In conjunction with issue #55, it will be possible to serve the pretty-printed js in development and the minified js in production.