Add image class to HTML figure. Closes #3928.

[madgen]

Add the class attribute associated with the image to the enclosing figure with a -figure suffix as suggested by @mb21. Closes #3928.

Example input:

![a](b){.myClass1 .myClass2 #id}

HTML output without the change:

<figure>
<img src="b" alt="a" id="id" class="myClass1 myClass2" /><figcaption>a</figcaption>
</figure>

HTML output with the change:

<figure class="myClass1-figure myClass2-figure">
<img src="b" alt="a" id="id" class="myClass1 myClass2" /><figcaption>a</figcaption>
</figure>