(keys) Support Vim motions

[sectore]

From README:

To enable Vim motions key binding, run with --vim=on once. It will be stored as part of settings, so only running once is needed. To disable, run --vim=off.

Vim motions: https://vimdoc.sourceforge.net/htmldoc/motion.html#up-down-motions

@CX330Blake Feel free to double check / review.

Closes #160

[CX330Blake]

LGTM! Also super thanks for the blazingly fast response.