Feature/grep enhancements

This is a new feature required for fedora-infra/datagrepper#118

Justification:  now that we have koji messages from the secondary arch instances, we have a ton of spam on the bus.  It would be nice to be able to make datagrepper queries saying "give me everything *but* the ``buildsys.tag`` and ``buildsys.untag`` messages.  This is specifically wanted for the new fedora-packages+datagrepper integration like at https://apps.fedoraproject.org/packages/fedora-release