Added a chat history plugin.

The mod_chat_history plugin provides chat history for all public chat messages.

Can be configured in the following ways:

- history_max: max number of messages stored in history
- history_default: the default number of messages to be returned when invoking !history
- history_connect: if > 0, then this number of messages is automatically sent when connecting to the hub

Removed the built-in !history command in favour of the mod_chat_history plug-in.

Make sure we unescape all chat messages before forwarding any of them to plugins.

Update example plugins.conf in documentation directory.

src/core/commands.h

@@ -54,10 +54,10 @@ enum command_parse_status
  */
 struct hub_command
 {
-	enum command_parse_status status;   /**<<< "Status of the hub_command." */
 	const char* message;                /**<<< "The complete message." */
 	char* prefix;                       /**<<< "The prefix extracted from the message." */
 	struct linked_list* args;           /**<<< "List of all parsed arguments from the message. Type depends on expectations." */
+	enum command_parse_status status;   /**<<< "Status of the hub_command." */
 	command_handler handler;            /**<<< "The function handler to call in order to invoke this command." */
 	const struct hub_user* user;        /**<<< "The user who invoked this command." */
 	void* ptr;                          /**<<< "A pointer of data which came from struct command_handler" */