Add heading-focused outlines and :heading

This makes a number of fairly big changes:

* Introduces a heading and heading level concept.
* Replaces the outline algorithm with a document headings concept.
* Requires document headings to not skip heading levels and start
  with heading level 1.
* Introduces a :heading pseudo-class selector.
* Introduces a :heading(level) functional pseudo-class selector.
* Does away with the section concept (except insofar it's needed to
  influence the heading level of h1/hgroup).
* Does away with sectioning roots.

Tests: ...

Fixes #83.