Add "message of the day" support to the broadcast plugin

Add MOTD ("message of the day") support to the broadcast plugin. This is a feature in jabberd that is supported by several other servers. It's not documented very well anywhere that I can find. However, the following was taken from the ejabberd docs, which seems to describe the feature fairly well:

example.org/announce/all – Message is sent to all registered users at example.org. If the user is online and connected to several resources, only resource with the highest priority will receive the message. If the registered user is not connected, the message will be stored offline (if oflline storage is available).

example.org/announce/online – Message is sent to all connected users at example.org. If the user is online and connected to several resources, all resources will receive the message.

example.org/announce/all-hosts/online – Message is sent to all connected users at every virtual host. If the user is online and connected to several resources, all resources will receive the message.

example.org/announce/motd – Message is set as MOTD (Message of the Day) and is sent to users at example.org as they login. In addition the message is sent to all connected users (similar to announce/online resource).

example.org/announce/motd/update – Message is set as MOTD (Message of the Day) and is sent to users at example.org as they login. The message is not sent to all connected users.

example.org/announce/motd/delete – Any message sent to this JID removes existing MOTD.