FR: Support configurable key mapping

[indiscipline]

This is certainly an important feature.
Tetronimia can't have them all enabled at the same time: we already have a PR #4 that I'm not totally fond of, as it breaks the ergonomics of the default key mapping (left hand on D and F and Space on qwerty keyboards).

There's a few options:

1. Support specific key map as some kind of a complicated switch. As far as I understand, cligen doesn't support parsing tables (or other key-value pairs) yet, so we need to resort to parsing long user input which is error-prone.
2. Support supplying a key map from a specific config file as a switch. Much more convenient. Tetronimia could generate a default file with self-explanatory syntax per request too.
3. Support a few default keymap modes, such as vim | emacs | wasd | .... Even easier (to support and to use), but not flexible enough.
4. Support overall config files. This is desirable, but complicates things a lot, encourages replay and discourages using the defaults. Having the support for configs leads to having no excuse to not implement some other features which is not exactly a welcome outcome :) This also requires a separate FR.

Please, share your thought on options 1-3 or other ideas on how to implement this elegantly.

[indiscipline]

Fixed in v0.3.0! See release notes for details and usage.

Went with the combination of options 1 and 3 of the original FR. That was fun. Unfortunately, now we're up one dependence (illwill) but it looks fine.

@kaushalmodi hope you'll like it. The emacs bindings are based on your #4.