irkerd: Fix -i / --immediate handling, and add a message argument

irkerd: Fix -i / --immediate handling, and add a message argument

This fixes some problems with -i/--immediate parsing in ccd311c5
(irkerd: Transition from getopt to argparse, 2014-03-06).

> +    parser.add_argument(
> +        '-i', '--immediate', action='store_const', const=True,
> +        help='disconnect after sending each message')

This does not match the old syntax where -i took two arguments (an IRC
URL and a message).

> -    if immediate:
> +    if args.immediate:
>          irker.irc.add_event_handler("quit", lambda _c, _e: sys.exit(0))
>          irker.handle('{"to":"%s","privmsg":"%s"}' % (immediate, arguments[0]), quit_after=True)
>          irker.irc.spin()

immediate should be args.immediate, and arguments[0] needs to be added
as a new (optional) positional argument.

This patch fixes both issues.