Support additional js directories

[joescii]

In order to make this work well for applications written with CoffeeScript, I would like to make this plugin support having additional js source directories. This will allow the sbt-closure plugin to pickup js files produced from compiling CoffeeScript, while keeping the jsm files housed in the src directory.

[eltimn]

If you want to add this, be my guest. I don't really have time to work on this. It needs some other updates as well, though. I think the compiler it's using is pretty old.

You may want to check out untyped's sbt plugins. It looks like they support coffeescript now.

[joescii]

The untyped plugins look promising. I will definitely check that out first. If I find my way back to sbt-closure, I plan to update the compiler while I have the hood up.