Allow use of ctrl + c to exit the repl through an environment variable

[davidsharp]

If QUIT_W_C (quit with c) variable exists, switch the exit command from CTRL+D to CTRL+C

There's definitely a nicer way to do this, but it means that an alias can be set up for example, and that ctrl+d remains the default.

Personally, I'm in the habit of hitting ctrl+c a few times, and doing the same with ctrl+d often ends in accidentally terminating my bash session, so being able to switch it to ctrl+c is preferable.