Switch to figures for block-level images

[davidmerfield]

Previously:

![Caption](/path/to/_image.jpeg)

Would become the following HTML:

<p><img src="/path/to/_image.jpeg" width="250" alt="Caption" /></p>

Now it becomes this:

<figure>
<img src="/path/to/_image.jpeg" width="250" alt="Caption" />
<figcaption aria-hidden="true">Caption</figcaption>
</figure>

• Use title rather than alt text when present for the caption
• Hide the figcaption unless image captions are enabled
• Tell Audun
• Tell Robin