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

The current line already has some assumptions that all but guarantee you're in a git repo (e.g. `node_modules`). What about

```bash
if [ -d ".git" ]; then
  git ls-files | grep -E *.coffee ...
else
  find . -type f -path "*.coffee" | grep -v node_modules ...
fi
```