Support for Python Markdown extensions

[rybesh]

Especially for skos:example values, it would be great if it were possible to take advantage of Python Markdown extensions, specifically fenced code blocks and codehilite. This would allow the generation of syntax-highlighted examples.

[nicholascar]

Hi @rybesh could you please put in a PR for this? I'd love to have the extensions support but have a few more critical issues to deal with at the moment, like adding ASCCIIDOC support.