Implement figure numbering for HTML output

This is a first attempt at implementing a `--number-figures` option for HTML output (https://github.com/jgm/pandoc/issues/615). This is a first step towards allowing figure references within a document.

There are a few questions that remain outstanding,

 1. How should other writers (especially the LaTeX writer, which already implements figure numbering) treat this option
  2. What form should the figure number take (e.g. a global counter for the entire document, as this implements, or just within the current (sub?)section
  3. How should the figure number be rendered? Currently it's placed in a `<span>` labelled with `class="figure-number"`