User configurable keybindings

Adds the ability to specify keybindings via the `rc.json` file.  Mainly done because if I'm going to use xo I'd prefer more mac-ish keybindings, and while I could just fork the project and modify, it seems like something that other people might want to do.

Feel free to reject this if you think this is out of scope: I'm not sure that I'd use xo myself except maybe as the editor to use when writing git commit messages (but it does seem useful for that).