Feature request: Customizable key bindings using a config file

[cibinmathew]

Enable debugger.py to read the key bindings and action mappings from a config file.

From a quick look, I think below could be modified https://github.com/inducer/pudb/blob/ba31011c8f491bd6c36b59636f93ebb297ccc180/pudb/debugger.py#L2060-L2075

[inducer]

It's not hard to get the key bindings from somewhere configurable. I would probably not be supportive of having UI to configure that (too much clutter), but having them in the config file seems halfway reasonable.

If you'd like to work on that, I'd be OK considering a PR.