Add support for vi mode keybindings

[cohml]

For posterity, I'm coming here from inducer/pudb#633 (paging @inducer)

So far as I can tell, urwid_readline does not support vi mode keybindings. However, this similar module from a different project suggests it can be done with something similar to what you've already implemented with ReadlineEdit.

This would be a very welcome and awesome addition to your package.