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/bot.py

@@ -33,6 +33,13 @@ class Bot:
     def plugins(self) -> Sequence[plugin.Plugin]:
         return self.__plugins
 
+    @property
+    def joined_channels(self) -> Set[str]:
+        """
+        Returns a list of all channels that this bot has joined.
+        """
+        return self.__channels
+
     def quit(self):
         self.__quitting.set()
 
@@ -67,6 +74,8 @@ class Bot:
         for ch in self.server_config.all_channels:
             msg = Message(None, None, "JOIN", ch)
             conn.send(str(msg))
+        # on_connect event on all plugins
+        await asyncio.gather(*[plugin.on_connect(conn) for plugin in self.plugins])
 
     async def on_join(self, conn: IrcProtocol, message: Message):
         log.debug("%s", message)

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