Feature: Item child callbacks

[flynow10]

Feature: Item child callbacks

Why

I was not able to find a good way of dynamically updating items in a menu based on the props passed to the show() function.

Proposal

Allow users to pass a function that returns a ReactNode to the Item component as a child.

<Item>
   {(args: HandlerParams) => {
      return <span>{args.props.key}</span>;
   }}
</Item>

[flynow10]

This is possibly related to #200