Add "[off]" prefix for messages not to be sent to the "other side".

If an in-game message begins with "[off]", it's not sent to IRC,
and vice-versa.

README.txt

@@ -109,6 +109,9 @@ Once the game is connected to the IRC channel, chatting using the 'T' or
  by anyone. Also, when someone sends a message to the channel, that text
  will be visible in-game.
 
+Messages that begin with "[off]" from in-game or IRC are not sent to the
+ other side.
+
 This mod also adds a few chat commands:
 
     /irc_msg <nick> <message>

src/callback.lua

@@ -21,6 +21,7 @@ end)
 minetest.register_on_chat_message(function(name, message)
 	if not mt_irc.connected
 	   or message:sub(1, 1) == "/"
+	   or message:sub(1, 5) == "[off]"
 	   or not mt_irc.joined_players[name]
 	   or (not minetest.check_player_privs(name, {shout=true})) then
 		return

src/hooks.lua

@@ -109,6 +109,8 @@ function mt_irc.hooks.channelChat(user, channel, message)
 
 	if mt_irc:check_botcmd(user, channel, message) then
 		return
+	elseif message:sub(1, 5) == "[off]" then
+		return
 	elseif foundchat then
 		mt_irc:sendLocal(("<%s@%s> %s")
 				:format(chatnick, user.nick, chatmessage))