Adds Atom support

This adds support for Atom, the package was converted from the Textmate bundle using their CLI tool. It still needs to be published, but I believe you should only need to do the following after installing Atom:

```
apm publish minor
```

[Everything that does is listed here](https://atom.io/docs/latest/publishing-a-package), it’s pretty tied to git and will update the tag for you as well, which will hopefully work fine alongside the Textmate and Sublime bundles.