Add option to check in to MM with an external command
    
    Since the internal checkin code isn't baked yet, let's allow
    people to use an external command (i.e. report_mirror) to check
    in. This is better than just calling report_mirror after q-f-m
    from your cron job because that would run the (rather heavy)
    directory scrape and checkin process even if there were no
    changes. q-f-m exits earlier than this if there are no changes,
    so it will only check in if there are changes.