Add graceful exits

This sets up a set of ropes and pulleys that signal the `Bot.keepalive`
function to clean things up after a quit signal has been sent. This
allows plugins to define an `on_unload` function to save any important
datas on intentional exit.

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_unload(self, conn: IrcProtocol):
+        pass
+
 
 def load_plugin(server_config: ServerConfig, plugin_config: PluginConfig) -> Plugin:
     name = plugin_config["module"]