Unit-level filtering for journald-ingestion

[tazjin]

(This issue was aprilabank/journaldriver#9, filed originally by @reiinakano)

Some users of journaldriver wish to opt out of forwarding every single log message that is ingested by the journal (which is currently the only setting journaldriver has).

My first thought about this is that there should be unit-level inclusion and/or exclusion lists for individual systemd units. This would let users say something like:

# only forward logs from X and Y
include:
  - x.service
  - y.service

or alternatively

# forward everything, except the `verbose_nonsense`-service
exclude:
  - verbose_nonsense.service

Now that priorities are forwarded, maybe filtering by priority is also an option.