Add a couple crude debug trace tools.
Another crude debug trace tool.
Much better grammar for headings and hr. Closes #26.

- Simpler.

- Faster enough that the "too good to be true" timing in perf-test.rkt
  needs to be lowered from 2000 to 1000 on my machine.

- Fixes issue #26.
Add - and = to the tokens.
Change some uses of `notFollowedBy` to `many1Till`.

This isn't possible when you _don't_ want to consume the `close` (as
`many1Till` does) and really do need only "negative look-ahead". But I
had some cases where I was going to consume the close, anyway, and in
that case it's simpler to use `many1Till`. Maybe faster, too, although
I'm not sure.

Also: Some DRY on code to eat trailing space, duplicated among HTML
parsing functions.