fig, table-wrap & caption Divs for JATS writer

[hamishmack]

Support writing <fig> and <table-wrap> elements with <title> and
<caption> inside them by using Divs with class set to on of
fig, table-wrap or cation. The title is included as a Heading
so the constraint on where Heading can occur is also relaxed.

Also leaves out empty alt attributes on links.

[hamishmack]

This adds a bit of code duplication in blockToJATS with the version of the function that supports titles starting with fig. Do we still need that version or is it ok to only support using a Div? If we do I can refactor out the code duplication.

[jgm]

Yes, we still need to support turning a Para containing just an image whose title starts with fig: into a figure. That's the standard (though hackish) way of representing a figure in pandoc.

[jgm]

Great, thanks.