Custom final message on keyboard interrupt

[denizs]

This PR introduces an additional kwarg named keyboard_interrupt_msg, which let's you customize the output of whaaaaat when a user hits ctrl+c. Now hardcodede as Cancelled by user, this let's you do something like this:

prompt(questions, style=style, keyboard_interrupt_msg="Ok ok, I'm backing off! See ya later")

which would result in:

[?] Some question where you hit ctrl+c

Ok ok, I'm backing off! See ya later

[markfink]

I understand that you want to add a lot of additional features to whaaaaat but this is not what we intended. whaaaaat is supposed to be a small library (like a building block). If you want to implement more complex dialogues I suggest you use something like https://github.com/finklabs/banana or write your own.