Add support for custom widget options

[n1k0]

We should add support for options to custom widgets, so these could access them as props.

const CustomWidget(props) {
  const {value, onChange, options} = props;
  return <div className={options.className} />;
}

const uiSchema = {
  "ui:widget": {
    widget: CustomWidget, 
    options: {
      className: "toto"
    }
  }
};