Add OnJoining module hook.

This patch adds an OnJoining module hook that allows a module to allow
or disallow joining any given channel.

modules/modpython/znc.py

@@ -281,6 +281,9 @@ class Module:
     def OnKick(self, OpNick, sKickedNick, Channel, sMessage):
         pass
 
+    def OnJoining(self, Channel):
+        pass
+
     def OnJoin(self, Nick, Channel):
         pass