Detect message from ZNC's privmsg module and behave gracefully

I'm using the (third party) ZNC "privmsg" module from http://wiki.znc.in/Privmsg and noticed that Textual doesn't detect its messages and behave appropriately (although the wiki page suggests that it has done so in the past).

This is a first run at implementing it - seems like a fairly simple fix and some light testing suggests it works, but I wanted to request some feedback before I dig further.