Accessiblity enhancements

• Add aria-required for required elements Feature request: aria-required="true" when a field is required #230
• Using aria-invalid="true" for form elements that have an error.
• aria-live attribute for error message section Meta: Accessiblity Enhancements (ARIA) #706 (comment)

These might be masked under an accessible/aria option (I know you don't like adding more to the API) or as an optional extend method.

/cc @pjackson28