* fix unclosed pi in code span
* fix unclosed dec in code span
* fix unclosed tag in code span

Closes #1066.

This reverts part of 2766698 and re-implements handling 
of tails in the same manner as the core.

Also, ensure line_offset doesn't raise an error on bad input
(see #1066) and properly handle script tags in code
spans (same as in the core).

Fixes #1068.

Ensure that start/end tag handler does not include tags in the previous
paragraph.

Provide special handling for tags like hr that never have content.

Use sets for block tag lists as they are much faster when comparing
if an item is in the list.

Fixes #1053.

* Ensure unclosed script tags are parsed correctly by providing a workaround for https://bugs.python.org/issue41989.
* Avoid cdata_mode outside of HTML blocks, such as in inline code spans.

Fixes #1036.

The HTML parser has been completely replaced. The new HTML parser is built on Python's html.parser.HTMLParser, which alleviates various bugs and simplifies maintenance of the code.

The md_in_html extension has been rebuilt on the new HTML Parser, which drastically simplifies it. Note that raw HTML elements with a markdown attribute defined are now converted to ElementTree Elements and are rendered by the serializer. Various bugs have been fixed.

Link reference parsing, abbreviation reference parsing and footnote reference parsing has all been moved from preprocessors to blockprocessors, which allows them to be nested within other block level elements. Specifically, this change was necessary to maintain the current behavior in the rebuilt md_in_html extension. A few random edge-case bugs (see the included tests) were resolved in the process.

Closes #595, closes #780, closes #830 and closes #1012.