Add late ping support

This lets one say `crowbot: ping Manishearth <message>` or `crowbot: silentping Manishearth <message>`, and the bot will ping the user (in the channel or in a PM respectively) when they next join.

If there are more than 5 stored pings for a user, they all are sent via PM.

Currently this doesn't check if the user is already in the room, I'm not sure if this is necessary.