Make it easy to pass className props down

[djgrant]

Instead of writing:

export const MyComponent = (props: { className: string }) => {
  return (
    <div className={props.className}>
      <div className="pt-10 pb-14 px-8" />
    </div>
  );
};

I could write:

export const MyComponent = classy.div(() => {
  return <div className="pt-10 pb-14 px-8">;
});

Or:

const MyDiv = classy.div();

export const MyComponent = MyDiv.extend(() => {
    return <div className="pt-10 pb-14 px-8">;
});

This would introduce two API changes:

1. If the argument type is a function, classy creates a wrapped component.
2. Classy components can be extended by calling an extend method. If a classnames object is the argument, it is merged with the original classnames object. If a function is the argument, the original component is wrapped, as per 1.