Make sure Grunt knows to compile Literate CoffeeScript too

No idea if this is a legit problem or not, or whether it’s just me missing something obvious, but the default Gruntfile provided by this generator didn’t seem to be picking up any files with `.litcoffee` or `.coffee.md` extensions. It looked to me like the Gruntfile template itself needed updating in order to support this, so this pull requests adds `.litcoffee` and `.coffee.md` extensions to the search paths for CoffeeScript files. Please let me know if there’s a better way to make this work, or if there’s another place I should be submitting this instead :grin:.

Also, the tests for this package seem to only really care about which files the generator places when you run it, so I wasn’t sure if it’d be in-scope to test whether the generated Gruntfile actually compiles the desired extensions or not. Let me know if that’s something that I should try and add in here too (my knowledge of how to test stuff like that in node is pretty much non-existent though… :sweat_smile:).