New feature for behavior between to shouldUnregister: true and shouldUnregister: false

[Hengry]

Is it possible to add a new setting which behave just like shouldUnregister: false, but will only return the input showed in the screen when submit/watch/getValue. Maybe called shouldUnregister: 'onSupervise' or something else has the same meaning.

The benefit of this idea are:

1. Input Value would be reserved in conditional rendering, which is the advantage of shouldUnregister: false.
2. User would not need to pick out the values not showing on the screen, which is the advantage of shouldUnregister: true.

And also I think the maintenance of this feature might be easier than shouldUnregister: false.