SketchFab Media links

Hello there

Do you think you could add SketchFab to your Media Extensions?
So you could have:

![Title goes here](https://skfb.ly/6stSI)

The embed code for SketchFab is much longer then YouTube though, for the above model the embed HTML is:

<div class="sketchfab-embed-wrapper">
    <iframe title="Title goes here" width="640" height="480" src="https://sketchfab.com/models/1e03509704a3490e99a173e53b93e282/embed?autostart=0&amp;ui_controls=1&amp;ui_infos=1&amp;ui_inspector=1&amp;ui_stop=1&amp;ui_watermark=1&amp;ui_watermark_link=1" frameborder="0" allow="autoplay; fullscreen; vr" mozallowfullscreen="true" webkitallowfullscreen="true"></iframe>
    <p style="font-size: 13px; font-weight: normal; margin: 5px; color: #4A4A4A;">
        <a href="https://sketchfab.com/3d-models/the-rosetta-stone-1e03509704a3490e99a173e53b93e282?utm_medium=embed&utm_source=website&utm_campaign=share-popup" target="_blank" style="font-weight: bold; color: #1CAAD9;">The Rosetta Stone</a>
        by <a href="https://sketchfab.com/britishmuseum?utm_medium=embed&utm_source=website&utm_campaign=share-popup" target="_blank" style="font-weight: bold; color: #1CAAD9;">The British Museum</a>
        on <a href="https://sketchfab.com?utm_medium=embed&utm_source=website&utm_campaign=share-popup" target="_blank" style="font-weight: bold; color: #1CAAD9;">Sketchfab</a>
    </p>
</div>

So you might have to trim this down.

SketchFab also has a REST API, so it could be possible to HTTP.Get and await the user name, description and other information of the model.

What do you think?

Start with src/Markdig.Tests/Specs/MediaSpecs.md and compare the existing media-extension cases with the SketchFab Markdown example. Determine the minimal expected SketchFab embed output rather than implementing the optional REST API, then document and verify the resulting behavior in the media specifications.