Enable descriptive header IDs.

Enable blackfriday.EXTENSION_AUTO_HEADER_IDS to generate the name of the
header ID from the text in the header. Works for prefix and underline
headers.

- TOC extraction had to be modified to look for `<li><a href="#`>
  instead of `#toc_` because of this change.
- Fixed a number of tests that depended on the presence of `toc_` with
  as an `id` or as a `href` value.