Output ^C when the user presses Ctrl-C

This gives the user some indication of what happened. If there's a ^C
somewhere on the previous line, there's a good chance the line wasn't
actually executed.
Add SetCtrlCAborts

SetCtrlCAborts can be called if the application wishes Prompt to abort
with an error when the user presses Ctrl-C (instead of restarting the
line input).

Inspired by pull request #30