Still unwieldy, but I'm working on it.

- Now putting chapters inside "<article>".
- Doing rendering of context.md + chapter markdown, instead of rendering
  the whole book at once.
- messed about with CSS rendering of headings

Also adds a few dependencies to Makefile, comments out some formatting
in render.pl, sets up a demonstration of converting a program from
specific -> general, perhaps lays groundwork for a discussion of the
utility of diff(1) in software terms, which seems like a good transition
back to extant literary tooling.