Add quit command

This is a working proof of concept for allowing the user to quit from the CLI without having to send a SIGINT to the process. I think there's definately a much better way to do this, for instance this implementation has to hook in to the app.py and duplicates code for handling shutdown clean-up found elsewhere i.e. __init__.py. 

@BjarniRunar  it would be great to get a suggestion / your thoughts about this.