A check has been added to know if args has an attribute 'callback'.
A check has been added to know if args has an attribute 'callback'.

If args has no attribute callback, the help text should be printed. Because
there was a check to::
        if args.callback is 'x':
It would raise an error before the parser.print_help() was reached.

mexicarne/tmuxp,thomasballinger/tmuxp,tony/tmuxp,thomasballinger/tmuxp,mexicarne/tmuxp