Ability to select a backend by content type

The MLM administration page at /admin/user/mlm/settings should have the ability to select/enable backends per content type. Currently you can enable multiple back ends, and you can select multiple content types to use at mailing lists, but you can not map a specific backend to a specific content type. When selecting multiple back ends, all content types default to one backend type.

As an example, when enabling the Drupal and Ezmlm-idx backends concurrently, all content types enabled default themselves to the Drupal backend on the node creation form.