Advanced background: Render the image's alt text to <figcaption>

[yhatt]

For accessibility improvement, when defined the alternative text for Marpit's background image ![bg](), <figure> element becomes to have a text label by <figcaption>.

![bg Alternative text](https://example.com/background.jpg)

<figure style="background-image:url('https://example.com/background.jpg');">
  <figcaption>Alternative text</figcaption>
</figure>

Also added the style for <figcaption>. It hides the caption text from screen, but keeps readable by screen readers.

Resolve #368.