[Suggestion] Implement "Private" messaging for comments plugin

Hey again,

I just had an idea - given that you've recently implemented creation of rooms for comments plugin (which is quite cool), I think now it would be more or less trivial for you to implement "private" messaging (I say that in quotation marks, because channels are still technically public). You can hardcode the slash command with the syntax /invite <user id> <room name>, which would then trigger the API call as if both the inviter and invitee had created the same room. Alternatively, a smoother approach for this could be not triggering any API calls, but just display a system message of a sort to the invitee that they were invited to a room, and they would have a choice if they want to join the room or not.