Ability to make images into hyperlinks

I have a use case where I need to make an image into a clickable link. It's not immediately clear how to do this in a flexible way (i.e. so it could apply equally well to text elements in a slide), but I have a working patch to allow it for an image. If you have suggestions on how it could be implemented more generally, I will be happy to update the PR.

Usage is simply an option `link` passed to the `addImage` function in its options hash. The value of `link` is the URL that the image should link to, e.g.

`slide.addImage(..., { "link": "http://www.google.com" });`