repl shortcut function

An idea from https://github.com/jonathanslenders/python-prompt-toolkit/issues/519#issuecomment-318978114 and https://github.com/jonathanslenders/python-prompt-toolkit/issues/519#issuecomment-320065017.

It could be useful to have one higher level of abstraction in shortcuts.py, for the full "main loop" of a prompt REPL. Something like

def repl(execute_callback, message='', **kwargs):
    while True:
        try:
            res = prompt(message, **kwargs)
        except KeyboardInterrupt:
            print("KeyboardInterrupt", file=sys.stderr)
            continue
        except (EOFError, SystemExit):
            break
        execute_callback(res) # Perhaps the callback should also be passed the cli instance

It could also support other features like keeping track of prompt numbers, or an input "queue" (see https://github.com/jonathanslenders/python-prompt-toolkit/issues/519#issuecomment-318978114).

Start by reading shortcuts.py and the two linked discussions from issue #519. Clarify the intended scope of the REPL abstraction, including callback handling, interrupts, exits, prompt numbers, and an input queue. Done means the API and supported behavior are agreed and implemented with coverage for the selected features.