notification: Allow specifying multiple scripts
    
    The notification hooks can be useful for doing other things than just
    announcing status on message bus. For this to be truly usable, we need
    the ability to use multiple scripts.
    
    This patch allows the command line option to be specified multiple
    times. Each given script will be called. Even if the script fails, it
    does not block the compose.
    
    Additionally the output of the notification scripts is logged now to
    make it possible to debug possible failure.
    
    Relates: https://pagure.io/pungi/issue/650
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>