Handle image tag wrapped in html

[zerolab]

<div class="illustration-description">
![The image's alt text here](https://example.com/image.png)
</div>

doesn't seem to be converted correctly

[engineervix]

As I was reading through the md_in_html extenstion docs I realised that this won't work because it's missing the markdown="1" attribute. It should work if it's

<div class="illustration-description" markdown="1">
    ![The image's alt text here](https://example.com/image.png)
</div>