Modify numbering frontmatter to customize "reset after" behavior

[fwkoch]

Currently, the numbering object in the frontmatter does not allow resetting figure numbering on each heading_1.

In general, everything resets at the page level (unless continue: true, then it does not reset). There are a few special cases: headings reset when the heading level above increments and sub-figures reset when figures increment. But this could easily be made more generic with something like:

numbering:
  figure:
    reset: heading_1

I suspect this change might also clean up the code and remove special cases around headings and subfigures.