Add hooks for external modules on subscribing and unsubscribing from lists

I need to perform actions when a user subscribes to a mailing list, or when the user unsubscribes from one.

This patch simply adds two hooks, mailchimp_subscribe_user($list, $email) and mailchimp_unsubscribe_user($list, $email), called in the mailchimp_subscribe_user() and mailchimp_unsubscribe_user() functions. It might be useful to others.