Consider using `<figure>` elements to preserve figure captions

Right now, figure captions are converted to simple image with a separate paragraph of text under it. E.g. 

```md
![](image src)

Image caption
```

 It would be nice if it was preserved that the text was an image caption, by wrapping it in a `<figure>` element:

```md

<figure>

![](image src)

<figcaption>Image caption</figcaption>

</figure>

```