Add ability to change the toggle key

[vutran]

Sometimes you already have cmd + space already registered to another app. Allow a way for the user to change their key bindings to another combination.

[adnasa]

Would this be pure configuration (as in config on file-format) or are you considering a preference menu (like every other app out there)? 😉

[vutran]

I'm open to both options. A preference window would probably make more sense if we want to target non-developers. If it's preference window, I was considering using my react-jsonschema to scaffold the forms but don't want the material-ui that comes with it.

See #28 for more reference on managing prefs/configs.