* another pass at the emmatcher to make certain that
    pathological indentation won't produce bad xhtml
  * fix a bug in autolink that made a hanging prefix
    generate a bogus `<a` tag.
  * restrict the depth of a ETX header to `<h6>`
  * write a manual page for callback functions.
  * don't emphasise `_` or `*` when they're in the
    middle of whitespace.
  * unescape trailing '  's when they're in the middle
    of a code section or `[]()` link.
  * tighten the parser for footnote links so that a
    construct like `[]-<]:` won't erroneously become
    a footnote.