Consider support for register_meta()

[WordPress 4.6 introduces a fundamental change to the way meta fields are registered.](https://make.wordpress.org/core/2016/07/08/enhancing-register_meta-in-4-6/) The primary change is the addition of meta metadata such as its type, description, value structure, and whether to expose it in the REST API.

It should be considered whether registered Fieldmanager meta fields should also call `register_meta()` in order to expose meta fields to this new meta API.