Added support for configuration options.

Users can now set the notifications_public_messages,
notifications_private_messages, and notifications_active_window Booleans
to specify which notifications they want to recieve.
Added support for indirect messages.

Previusly, the notifications plug-in only notified the user about
messages that were directly addressed to them. With this update, the
user can now set the notifications_indirect_messages Boolean to be
notified whenever a public message contains his nickname.