Add setting to use/override core's default field wrapper for un-configured fields

Yeah, this makes sense. If you don't specify anything in a field setting having to do with Fences, you should get default Drupal core behavior.

That way if someone adds Fences to an existing site, makes some new fields with Fences (maybe, say, in a brand new content type), and deploys that.... they don't have to worry that all their own crusty fields suddenly changed when they didn't tell them too.

So basically, there's a 'no setting' state (which uses the core markup with three/four divs), a 'div' Fences wrapper (that uses a div, but with very different markup from the core default), and a 'none' state that uses Fences markup that has no HTML wrapper at all. Could be a bit confusing to have these three states, but also, totally makes sense.