Various changes:

- Add HTML parsing (although I'm not distinguishing block and inline).
- `parse-markdown`: Use parameters for footnotes and references.
- Add `read-markdown`: Like the old main entry point.
- Tweak randomized testing to be bit more realistic.
Comments.
Provide parse-markdown and read-markdown
Footnote fixes.
Fix heading grammer: Need to eat trailing $spnl.
Delete unneeded space in footnote def.
Add <br> when two spaces before newline.
Various changes:

- Add `parse-markdown*` for internal use, doesn't set parameters like
  footnote number.

- Add `maybe-tighten`, which fixes up loose vs. tight lists (meaning
  the items are in `p` elements vs. plain text).

- Blockquotes need to parse their contents, not plain text.
Move `display-expr` and static file test to own modules.