Operator announcement command

Write a command that allows the bot operator to send an anannouncement to every guild its integrated with.

This is useful for announcing new versions and features. It should default to needing to be configured by each guild's owner - namely, the channel in which the announcements are to be delivered to.

Alternative to a command may be that on bot init, if a file exists corresponding to the version, that would be read and iteratively delivered. This would be a much more involved implementation though, as it'd require tracking these deliveries, and/or the first time the bot is initialized on each release version.