Add support for field handling

Alongside taxonomy vocabularies and user roles, fields are a commonly needed shared component. They differ from the above two in that they require both a field and a field instance attached to an entity type and (usually) a bundle.

Some relevant code is in the debut_media feature, though most of it should be left behind as it deals with features handling.

The first field handled probably should be field_tags, since it's related to the existing tags vocabulary handled already by Apps compatible.