Allow multiple keys for same action

[Friptick]

Related to #96

It would be nice to be able to attribute more than one key to a given action. For instance in .config/epy/configuration.json:

  "Keymap": {
    "ScrollUp": "k",
    "ScrollDown": '["j"," "]'

This particular config, for example, would make it possible to use the spacebar for scrolling (very convenient) while keeping that expected functionality on the j key too.

Thanks.