Added support for dynamic commands.

Dynamic commands are user commands that can be added dynamically to
the hub by a plugin.

The example plugin (mod_example.c) adds a !example command that when
invoked send a message to the user who invoked it.

src/core/hub.h

@@ -114,6 +114,8 @@ struct hub_info
 	struct linked_list* chat_history;    /* Chat history */
 	struct linked_list* logout_info;     /* Log of people logging out. */
 
+	struct command_base* commands;       /* Hub command handler */
+
 #ifdef PLUGIN_SUPPORT
 	struct uhub_plugins* plugins;
 #endif