Allow React nodes to be passed to caption and subcaption options

[benwigley]

This PR allows React nodes to be passed to the caption and subcaption options of the photo object.

This is useful for when you want to give credit to the author of the photo with a link.

{
  // other options...
  subcaption: (
    <span>
      Photo by
        <a href={photo.creditUrl}>
          {photo.creditName}
        </a>
    </span>
  ),
}