Good news! The game already has the ability to enable this. In the chat box where it says 'chat' horizontally, below that there is a little box with a >. Click that. Then select the channel settings tab and scroll down on the list to the left until you see alliance. Select it and then click on the option Flash Channel.
I suggest you check this feature out. A lot of things you can do in this options panel. See you in game!
If I always appear prepared, it is because before entering on an undertaking, I have meditated for long and foreseen what may occur.
--Napoleon Bonaparte, 1831