Add configuration option to avoid flooding channel with on_join function

[leifmadsen]

Currently whenever someone joins a channel, the on_join function is called, which can cause flooding to the channel. An option should be added to avoid spamming the channel, or maybe I've identified a bug, and the message should be messaged to the joiner privately instead of sending into the channel.

[gehaxelt]

Hmm... I thought that this event is only triggered when the bot joins the channel. However this might not be the case:

The method on_join will be called when a “join” event is created (which is done when the server sends a JOIN messsage/command)

I'll label this as a bug for now, but maybe we can use this behaviour to solve #30 by sending a private message to the newly joined user?

[h4ck3rm1k3]

yes the same