Move message size limit logic to base plugin

This is beneficial for all plugins so the bot doesn't accidentally
spam things because the plugin writer didn't check their inputs

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

omnibot/plugin.py

@@ -36,7 +36,7 @@ class Plugin:
         return self.server_config.nick
 
     def send_to(self, conn: IrcProtocol, who: str, message: str):
-        message = Message(None, None, "PRIVMSG", who, message)
+        message = Message(None, None, "PRIVMSG", who, message[:1024])
         conn.send(str(message))
 
     async def on_join(self, conn: IrcProtocol, channel: str, who: Prefix):