Added MessageHistory.pm and MessageHistory_SQLite.pm. May eventually port
and add MessageHistory_Hashtable.pm as was done with Quotegrabs, but this is
not particularly high on the todo list.
Antiflood.pm has been updated to use the new MessageHistory API.
The `recall` command has been moved from Quotegrabs into MessageHistory. It
also now has the ability to ignore messages containing the recall command
itself, for improved usability.
Likewise, the `grab` command will now ignore previous `grab` commands when
grabbing by regex in order to prevent accidentally grabbing previous grab
attempts.
The `join` and `part` commands have been improved to accept multiple channels,
and `part` will use the current channel if none is provided.
