Support form chunks use case

[insin]

Sometimes you just want an onChange which handles a chunk of form fields rather than encompassing the entire form., e.g. when rolling your own support for data held in objects or arrays.

You might also have nested AutoForm components to allow for specialised onChange handling, where only the top level form defines an onSubmit.

Allowing the component rendered by AutoForm to be customised and having its onChange call event.stopPropagation() should be enough to support this use case.