Add channels to messages framework

[Orcun Avsar]

Hi,

I don't use messages framework on a base template but have it in my installed apps because I need them on some particular pages. Some third-party apps (django-social-auth for my case) creates messages for itself. When I need to use messages in a single page I can't show all messages to user because I'm not sure how many messages are waiting in the queue for the user. Instead I want something like a channel whenever adding a message for a specific page (or purpose).  I think this can be done in a way without breaking backward-compatibility. 

Here is my case: I have a decorator that checks user's cart and redirects it back to "my cart" page with a warning indicating the cart has been updated. But, I can't show that message through messages framework because of the issue above.

If a channel is not given as parameter, messages framework may work as it is right now. This parameter is needed when both adding and retrieving messages.