Add extra extensions from the default Markdown set

I found that I required the 'toc' extension for migrating some of my content - so I added it to the extensions. While I was adding 'toc' I decided to add a few others that seemed safe and useful. They are all included in the default set.

Documentation on the default extensions:
http://pythonhosted.org/Markdown/extensions/index.html

This pull request adds 'toc', 'extra', 'sane_lists', 'wikilinks'.