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

If in a git repo then having the `git ls` is probably best (as you probably don't want to lint files not part of the repo). However I do think we should allow you to use this outside of a git repo.

I am fairly on the fence about whether we want to find the files in the Makefile or the script. I was trying to make the really easily implementable by having it in the script, but most of our Makefiles already prepare a list of tests and libs so that wouldn't add much work. 

We could of course do both. If no args, (find or git ls) else use given args?