Support for CoffeeScript Specs

Allow spec files to be written in CoffeeScript.

I felt that the `karma` plugin was the right place to have this, since one shouldn't need to include a `coffee` spell just to be able to execute tests written in CoffeeScript. Also, we don't build `*.spec.coffee` files into the `build` directory, so `spell-karma` needs to be able to process CoffeeScript directly from source.