Vim-like bindings in text areas.

Addresses #859 (and others).

Pulls in https://github.com/jakub-m/vim-in-textarea, as originally suggested by @kebot.

The approach is to have an activation keystroke `Control-Alt-V` (But we should consider making the particular keystroke configurable).  This seems better than having an option because, for many small text areas, we don't want to be pfaffing around with vim-like bindings.  For longer texts, like this one, it should be helpful to have vim-like bindings available.

Issues:

- Doesn't work on gmail/compose.  But nothing probably ever will, given the way that gmail works.

If somebody else could try it out and give some feedback, then that would be great.  Watch the HUD.  It gives (some) feedback as to the vim mode you're in.