Add on_connect callback and joined_channels property

* on_connect is called on all plugins on the first message received from
  the IRC server
* joined_channels property gets all of the channels that this bot has
  currently joined in IRC

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

omnibot/plugin.py

@@ -58,6 +58,9 @@ class Plugin:
     async def on_load(self):
         pass
 
+    async def on_connect(self, conn: IrcProtocol):
+        pass
+
     async def on_unload(self, conn: IrcProtocol):
         pass