Create an expanding streetview image with featherlight using Markdown

(originally posted in Slack: https://getgrav.slack.com/archives/C3Z7KN6MC/p1490592651037943 )

With the featherlight plugin, I can do this markdown: ![Alt text](local-image.jpg?classes=embed&lightbox=)

.. and Grav processes the classes and the plugin processes the necessary lightbox DOM manipulation etc

But with remote URLs (static Streetview URLs specifically), this doesn't work

e.g. ![Alt text](https://maps.googleapis.com/maps/api/streetview?size=640x250&location=-43.4952014,172.7113451&heading=0&pitch=-2&key=API_KEY&classes=embed&lightbox=)

(I have an API key BTW but not posting it here, so that URL won't work without a real key)

It half works, because the classes get pre-processed by Grav, but the featherlight stuff doesn't.

Does anyone know if this is a bug, or if there is a good workaround? I figure I can probably write a shortcode extension once I figure out the desired target markup. That's one workaround.