Add ability to mute unread count / mentions

[cblgh]

with the advent of the https://github.com/cabal-club/cabal-bot github integration, i found a need to mute notifications / new channel activity

that is, i want to be able to peek at the github channel, at the same time i don't want it to show that it has new messages, or that i have been mentioned in it. it's just a bit stressful / not useful for channels with a lot of activity and/or automated feeds as in the github integration case