Adds basic support for the Atom text editor

Hey! I converted this package from the TextMate so it’s ready for [Atom](http://atom.io).

I can also keep the Atom fork updated separately if you’d prefer. This is what I’ll be doing for the [OpenType feature bundle](https://github.com/brew/opentype-feature-bundle/pull/2), as Atom is pretty tied to tags on GitHub for its own package manager.

Thanks for all your work around Glyphs! :abcd: :sparkles: