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>