Allow to filter by journald priority

[adrianlzt]

Example:

check_logfiles.pl --type journald:unit=etcd,priority=err --warningpattern='.*'

To avoid merge conflicts, it has also the PR 26 because both PRs modify the same line.
Not sure if this is the correct way to handle this.

[adrianlzt]

Another example:

check_logfiles.pl --type journald:unit=etcd,priority=err..err --warningpattern='.*'

Just to use err messages.

Otherwise, it will use from that priority (included) to more important levels.