Markdown Parser Updates

The PHP Markdown Block Parser is out of date, as it is missing a few HTML5 tags. This causes certain block-level elements to ruin the valid HTML output. These include the ``<output>``, ``<figcaption>``, ``<object>``, ``<source>``, ``<track>``, ``<param>``, ``<math>``, ``<svg>``, ``<canvas>`` tags (and probably a few others that I forgot to mention).

Granted, these tags are not commonly used unless building a website that uses HTML5 technologies &mdash; still, eventually *someone's* going to notice, right?

I've only made modifications to the relevant portions of the Markdown_Parser class, as things like the Wordpress plugin are not used in Cindy. Perhaps in the future, unused code can be removed from the ``markdown-parser.inc.php``, as that will cut down on the overall size of Cindy.