Handle all unknown IRC command by forwarding the arguments as a message body

This way, the users can receive the result of any IRC command (although not
parsed nor formatted in anyway) when biboumi doesn’t support it

fix #2884

src/irc/irc_client.hpp

@@ -211,6 +211,7 @@ public:
    */
   void on_channel_mode(const IrcMessage& message);
   void on_quit(const IrcMessage& message);
+  void on_unknown_message(const IrcMessage& message);
   /**
    * Return the number of joined channels
    */