Delete markdown.pl stuff.
Break into smaller functions.
Initial commit using Parsack.
Add dependency on parsack
Fix blockquote. Refactor `try`.

- blockquote: Fix mistake in grammar causing nontermination when only
  the first line has a `>` but not subsequent lines.

- Re `try`: Make the use of `try` the choice/responsibility of each
  parser, because only each parser knows if it needs backtracking.
Replace <!> with notFollowedBy
Add footnote refs and defs.
Replace more <!> w/ notFollowedBy. Don't use <?>

<?> is N/A because we should never have errors.
Review use of `try`.

I'd done this for blocks, but missed some inlines. Again, the idea is
to localize the try with each parser, since only the parser can know
if it needs to look ahead and backtrack.
Disable $raw-HTML for now.
Add test: Parse random text.

Any input should parse successfully. In other words, any text is a
valid Markdown format file. Try random string and check there is no
parse error exception.
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.