mailchimp new functions

[mtuchi]

Summary

• listMembers(): https://mailchimp.com/developer/marketing/api/list-members/list-members-info/
• addMember(): https://mailchimp.com/developer/marketing/api/list-members/add-member-to-list/
• Add or update list member: https://mailchimp.com/developer/marketing/api/list-members/add-or-update-list-member/
• updateMemberTags(): https://mailchimp.com/developer/marketing/api/list-member-tags/add-or-remove-member-tags/
• deleteMember(): https://mailchimp.com/developer/marketing/api/list-members/delete-list-member/
• archiveMember()
• listAudienceInfo()
• listAudiences()

Issues

Reference #323

Review Checklist

Before merging, the reviewer should check the following items:

• Does the PR do what it claims to do?
• If this is a new adaptor, has the migration tool been run and the
  migration guide followed?
• Are there any unit tests? Should there be?
• Is there a changeset associated with this PR? Should there be? Note that
  dev only changes don't need a changeset.