Add a command mode and edit mode

[drewokane]

In using the gorilla notebook I have run into the already mentioned frustration of emacsy keybindings not working (on Mac at least). I thought a bit about this and experimented and had an interesting idea I thought I would suggest. Why not have a command mode, where the gorilla keybindings work for editing cells, moving things, saving the notebook, etc, and then an edit mode where "regular" keyboard bindings work, e.g. emacs keybindings for text editing. A preliminary search within Mousetrap suggests to me (at least from my limited experience) that we could pause and un-pause the gorilla keybindings. I'm willing to put in the time for this, with the caveat that I haven't worked in javascript before.

[benfb]

Feel free to open a PR for this if you get the time—otherwise it's probably not worth risking the simplicity of gorilla for beginners.

[drewokane]

PR has been added: #262

[benfb]

Ah, great thanks! That will actually help with #185 too.