Add a way to filter messages in rss feed

On the openSUSE Factory mailing list (and a couple of other mailing lists for other variants), we have a periodic announcement mentioning the new snapshot being released for openSUSE Tumbleweed. Those messages always have the same title and always start a new thread, so I was thinking that we could add type query parameter with thread value and title query parameter which does a regex check for titles fitting the value of it so that we can filter out only those announcements in the /feed/ endpoint