Implement different keybindings

[solkaz]

Description

Implement ability to select from the default keymap, Emacs, Vim or Sublime Text.

Motivation and Context

Fixes #557 Because it's flippin' awesome

Checklist:

• [x] My code follows the code style of this project.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

Browsers

• [x] Chrome
• [x] Firefox
• [x] Safari
• [ ] Mobile:
• [ ] Other:

Screenshots (if appropriate):

[solkaz]

For what? There are definitely people who would use these keybindings, including yourself.

[solkaz]

Because it was a small feature that took 10 minutes to implement yet would be appreciated by some people