Add options for automatically accepting pending values

[10xjs]

Some use cases may want incoming values to automatically replace current values. Add a nextValuesPolicy or similar field to useForm() config to control this behavior.

possible behaviors:

• accept: automatically accept all incoming values
• reject: automatically reject all incoming values (this would make the values prop behave as an "initialValues" prop)
• default: current behavior, each field derives a detached state and offers accept and reject callbacks
• acceptPristine: automatically accept all incoming values on fields that have not been edited and are not focused
• callback funciton: some kind of user defined callback, interface TBD