Add Member to a Segment after subscription

Hi! :)

I am using Mailchimp and Webform Mailchimp modules, and I wanted each of my forms to add the subscriber to a different Tag, after the subscription has been made.

In order to achieve this I added a small change to the Mailchimp module: a new parameter segment_id in the mailchimp_subscribe function, and a call to addSegmentMember after the successful addOrUpdateMember.

I am attaching patch. Please, tell me if there is something wrong with it or if you believe new changes should be made.