Allow model rendering to be overridden

[dantman]

This partially fixes #41 by allowing the way ModalProvider renders modals to be overridden.

e.g. A Material UI v5 project should be able to use this to make it compatible until MUI v5 is released and you make a new major for v5.

ReactDOM.render(
  <ModalProvider
    renderModal={params => {
      const { Component, props, key, handleClose, handleExited } = params;

      return (
        <Component
          {...props}
          key={key}
          onClose={handleClose}
          {...(handleExited && { TransitionProps: { onExited: handleExited } })}
        />
      );
    }}
  >
    <App />
  </ModalProvider>,
  document.getElementById('root')
);

Not sure how you want to document this.

[Quernest]

I closed this PR because merged the alternative.
#43