Sync the names of customer lists that a member is part of to Mailchimp

As discussed at Syncing customers & contacts to Mailchimp - #24 by jack-tavare, add a new field to Mailchimp to store the names of each customer list that a customer/contact is part of.

This would allow for more advanced list mailings in Mailchimp (and dynamic lists) without the need for paying for extra audiences in Mailchimp.