bot: add plugin option to receive all messages.

[wathiede]

Allows plugins to passively consume messages without being directly addressed.

[StalkR]

The command logic was intended for explicit, directed commands. For plugins who need to consume all messages, you can already use goirc's generic HandleFunc https://github.com/fluffle/goirc/blob/master/client/dispatch.go#L153. Some plugins already do that:

• https://github.com/StalkR/goircbot/blob/master/plugins/old/handlers.go#L63
• https://github.com/StalkR/goircbot/blob/master/plugins/scores/handlers.go#L79
• https://github.com/StalkR/goircbot/blob/master/plugins/sed/handlers.go#L42
• https://github.com/StalkR/goircbot/blob/master/plugins/urltitle/handlers.go#L58

I wonder if your use case could be addressed with this. Do you have a plugin example to show how your proposed changed is used?

[StalkR]

Replied on the CL directly, we can continue there.