Enable keybindings to be flexible

[netskink]

Feature Request 🚀

Is your feature request related to a problem? Please describe.

While trying to get pymakr and pytest plugins to work together, I found that there is a conflict with pytest test a file which is mapped to ctrl-alt t and pymakr ctrl-alt t which is toggle console. I also found that ctrl-r which is clear the output buffer is hard coded to be clearTerminal.

Describe the solution you'd like

I would like to write keybindings so that something like this in keymap.cson

atom-workspace':
  'ctrl-r': 'pymakr:clearTerminal'

Could be something like this

atom-workspace':
  'ctrl-w ctrl-x': 'pymakr:clearTerminal'
  'ctrl-w ctrl-t': 'pymakr:toggle REPL'

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Teachability, Documentation, Adoption, Migration Strategy

If you can, explain how users will be able to use this and possibly write out a version the docs. Maybe a screenshot or design?