Add on_load for plugins

This asynchronous function is called on all plugins right before the IRC
connection is made.

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

omnibot/plugin.py

@@ -51,6 +51,9 @@ class Plugin:
     async def on_message(self, conn: IrcProtocol, channel: str, who: Prefix, line: str):
         pass
 
+    async def on_load(self):
+        pass
+
     async def on_unload(self, conn: IrcProtocol):
         pass