Make style guide into npm installable package with shell script to lint

The part about node_modules only guards against the case where node_modules exists - it doesnt expect it.

I would actually be fine with making this script just take a list of files, filter out the ones that aren't coffee, and run the linter with our config. Finding the list of files could be left to the Makefile, since that will probably be repo-specific (though not linting files in node_modules seems pretty universal...)