Exchanged markdown.php by its respective version from Markdown Extra

As requested, I put in a pull request for the exchange of markdown.php from PHP [Markdown 1.0.1q](http://littoral.michelf.ca/code/php-markdown/php-markdown-1.0.1q.zip) to [PHP Markdown Extra 1.2.7](http://littoral.michelf.ca/code/php-markdown/php-markdown-extra-1.2.7.zip) in the dropplets/plugins directory.