Handle MOTDs

[zacstewart]

Handles three commands: RPL_MOTDSTART, RPL_MOTD, and RPL_ENDOFMOTD to store the MOTD in state and send an event whenever the whole MOTD has been received.

[zacstewart]

I was interested in firing events for all three commands as they come in as well, but I'm not sure how to map commands to events. Events in ExIRC are not well documented and I'm not sure there's a consistent naming scheme.

Personally, I think it'd be useful to have low-level events that closely map the name of IRC commands like RPL_STARTMOTD = :rpl_startmotd, and then high-level events for synthetic events like :received_motd when the whole MOTD is received.