Migrate to core config

After chatting with @lee-dohm, we decided to move from the previous config concept of the `overrides.cson` file and instead move to Atom's core config system. This pull request covers that migration.

Not satisfied with how this behaves in [settings-view](https://github.com/atom/settings-view), but we might be able to resolve that by pushing some upstream changes?

Overall a lot of code was removed, which is good. A lot of that code were tests though. Seeing as I'm the only user thus far, there's no need to waste effort on automagically migrating the old config file to the new scheme.