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.

* Add `language-` prefix to output when syntax highlighting is
  disabled for both codehilite and fenced_code extensions.
* Add `lang_prefix` config option to customize the prefix.
* Add a 'pygments' env to tox which runs the tests with Pygments
  installed. Pygments is locked to a specific version in the env.
* Updated codehilite to accept any Pygments options.
* Refactor fenced code attributes.
   - ID attr is defined on `pre` tag.
    - Add support for attr_list extension, which allows setting arbitrary 
      attributes. 
    - When syntax highlighting is enabled, any pygments options can
       be defined per block in the attr list.
    - For backward compatibility, continue to support `hi_lines` outside
      of an attr_list. That is the only attr other than lang which is allowed
      without the brackets (`{}`) of an attr list. Note that if the brackets
      exist, then everything, including lang and hl_lines, must be within
      them.
* Resolves #775. Resolves #334. Addresses #652.

Codehilite generates differant output with differant versiosn of pygments.
We support those various versions of pygments but can't unsure that the
same version is always available on every platform tests are run on.