Allow configuration of custom modes

[davelosert]

Currently, there is only the layoutMode. But the code is generic enough to allow the users to also define custom modes with their custom actions.

This would maybe also be a good time to make it configurable if a mode should be automatically exited on the first action.
Not having this would enable something like a resize mode, where h j k l or the arrow-keys could perform a resizing of the current window.