Add support for !tell

[rknLA]

Bot support for telling people messages when they're next in the channel.

Usage:
Assuming someUser is not in the IRC room:
!tell someUser hey, email me when you get a chance

Bot logs the message, and when someUser enters the room, the bot lets them know, Hi someUser, rknLA left you a message: hey, email me when you get a chance

I guess it's possible to combine the irc-slack-bot with another bot that does this, but it seems excessive to have so many bots in one channel, so might as well start rolling features into this one.. Maybe it makes sense to fork rdiobot off into its own thing to keep things tidy..