Make navigation in undo space easier

[paxperscientiam]

When using undo-propose, it would be great to navigate undo buffers with the arrow keys (for instance).

[jackkamm]

While I agree that adding undo keybindings (or a hydra) would be nice, I don't think the arrow keys would make sense without redo, which I don't plan to add anytime soon (#15). I'll have to think some more about what a good set of keybindings might be, and will leave this issue open in the meantime.