#38 AsciiDoc HTML breadcrumbs now include virtual TOC entries

• Best to review per commit.
• Notes about the changes:
  • The breadcrumbs are now based on the TOC, rather than on the page containment. This ensures consistency between the TOC and breadcrumbs.
  • The virtual TOC entries that are ancestors of the current page are now also included in the breadcrumb.
  • As before, the breadcrumbs end at the current page. Virtual TOC entries within the page are not part of the breadcrumb of that page.
  • Note that section headings on a page are never part of breadcrumbs, as they can be in the TOC, but can't have child pages.

Addresses #38