Use a prefix key to execute the external key handler

The first key pressed after the prefix key will be passed to the external key handler, unless the key is `Escape`, which is used to cancel the prefix. The default prefix key is `Ctrl-x`, and can be changed in `config.def.h`.

I think this approach is better than executing an external command *every time* an unmapped key is pressed.

Note that Ctrl-x also clears the number prefix, though I'm not sure if it's better than leaving it alone.