Support form chunks use case

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.