Markdown support

This pull request offers the [Markdown](http://daringfireball.net/projects/markdown/) support (using [this library](http://michelf.ca/projects/php-markdown/) as a git submodule) in link description: both on the website and in RSS/Atom feeds.

The user can choose to enable this functionality through two variables in the configuration array: `$GLOBALS['config']['PHP_MARKDOWN']` and `$GLOBALS['config']['PHP_MARKDOWN_EXTRA']`.

PHP Markdown Extra is an overlay to classic Markdown, you can discover its improvements [here](http://michelf.ca/projects/php-markdown/extra/).

This pull request doesn't include any CSS, users are free to implement their own or choose an [existing one](https://github.com/dhoko/ShaarliTemplate).