EditBox now uses urwid_readline library. Fixes #353

[danschwarz]

The urwid_readline library allows readline style key commands. Equivalent to readline with set -o emacs. ctrl-a, ctrl-e, ctrl-k, etc.
The library doesn't have a mode equivalent to set -o vi, unfortunately, but this will add functionality sufficient to fix 353.

[ihabunek]

Thanks, but I'm going to decline this one. Since tooi is coming along and aims to replace toot tui, I don't want to add new dependencies which are just used in the TUI, especially lesser known ones which are probably not packaged for various distros.