bot: stop plugin callbacks on the first success

When a plugin callbacks successfully handles an event it returns True,
and it now stops processing callbacks. It may be changed later to handle
multiple callbacks per event.

edmond/bot.py

@@ -155,4 +155,5 @@ class Bot(irc.client.SimpleIRCClient, Logger):
             callbacks = plugin.callbacks
             if etype not in callbacks:
                 continue
-            callbacks[etype](event)
+            if callbacks[etype](event):
+                break