Added option for hiding gifs

Here's a rough cut for the first option in #13 .

I found that Medium loads embedded gifs and Giphy links with an element with a progressiveMedia class, in which lives a preview thumbnail. If this thumbnail is a gif or a link to giphy, this hides it. This also hides the parent element, which is a aspectRatioPlaceholder.

I'd be interested in taking a look at the second implementation as well, but I figured I would start with this!