Rewrite mentioning channel name

[DarthGandalf]

Let's say the channel mapping is

"channelMapping": {
  "#discord-A": "#irc-A",
  "#discord-B": "#irc-B"
}

When #discord-A is mentioned on discord side, it would be nice to rewrite it to #irc-A, and vice versa

[Throne3d]

Hey! This project is mostly maintenance-only. I'll mark this issue as "help wanted" and "new feature" in case anyone wants to pick it up. With an especially complete PR (including tests), I'd be interested in merging this change.