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.

Fixes #1055.

* 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.

Fixes #897.

If the Python stack comes within 100 frames of the recursion limit,
then the nesting limit of blockquotes is met. Any remaining text,
including angle brackets, are simply wrapped in a paragraph. To
increasing the nesting depth, increase Python's recursion limit.
However, be aware that each level of recursion will likely result in
multiple frames being added to the Python stack. Therefore, the
recursion depth and nesting depth are not one-to-one.

Performance is an concern here. However, the current solution seems like
a reasonable compromise. It doesn't slow things down too much, but also
avoids Markdown input resulting in an error. This is mostly only a concern
with contrived input anyway. For the average Markdown document, this will
likely never be an issue.

Fixes #799.

* Python syntax upgraded using `pyupgrade --py3-plus`
* Travis no longer uses `sudo`. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

See #760 for Python Version Support Timeline and related dicussion.

Adjust pattern to allow for escaped hashes, but take care to not treat
escaped escapes before hashes as escaped hashes. Close #762.

Fixes #435.