Add support for JOIN event on IRC protocol

[julianolf]

On IRC protocol a join event is triggered whenever an user enters a channel. Capturing this event can be used to create nice plugins to interact with users or perform some routines just after they join the channel the bot is.

Looking in the documentation, more specifically in the events for callbacks section, it seems to be quite easy to do such thing.

So a new category of commands will be necessary, just like the passive commands but without a text message being sent as a parameter, just the channel and the new user's info will be available.