Fix support for data-uri

Per https://asciidoctor.org/docs/user-manual/#managing-images

> ...you can embed the images directly into the document by setting the data-uri document attribute.

We use encoded images for the "X-Pack" and "Edit_me" images (in https://github.com/elastic/docs/blob/master/resources/web/styles.css), so it would be nice to use this feature in asciidoctor.

Unfortunately, if I add an image like this:

> :compat-mode!:
> :data-uri:
>
> image::xpack-tag.png[X-Pack]

The image is omitted from the output of the current asciidoctor version of the documentation build.

However if I test Asciidoctor directly, it works as follows:

asciidoctor index.asciidoc -b xhtml :

``

asciidoctor index.asciidoc -b docbook45 :

`

X-Pack
`