Add Atom as an Editor option in Installfest

I wanted to add [Atom](https://atom.io/) as one of the Editor options in the Installfest because it's:
- Free & open-source
- Easy to install for OSX 10.8+ users
- Should feel relatively familiar to TAs & instructors used to helping with TextMate & Sublime
- Hackable with HTML, CSS, and JavaScript, which might be cool for RailsBridge grads looking for challenges to apply their new skills to.

With apologies for being a tiny bit of a company shill. :heart: :octocat: :octopus: