failed to add element/slide attribute which includes equal signs

[utensil]

If the value of an attribute includes equal signs, then this attribute is ignored.

One example of the use case is setting data-background-iframe to a URL with a query string:

<!-- .slide: data-background-iframe="https://www.youtube.com/embed/XXXX?rel=0" data-background-interactive -->

The reason of this seems to be in https://github.com/hakimel/reveal.js/blob/master/plugin/markdown/markdown.js#L285 , which requires the value of an attribute doesn't include " or =.

[mrmanc]

This prevents us from embedding published Google Drive documents as the URLs include = signs.