Add a list-unsubscribe header with the unsubscribe URL

It would be extra-nice if all the notifications sent via email from phpBB had the [list-unsubscribe]http://www.list-unsubscribe.com/ header set, pointing to the "unsubscribe" URL already provided in the body of the email.

Since all major email providers are supporting it, this displays a nice "unsubscribe" button right next to the "report spam" button.

Thanks,