Switch to Marked API

Switching over to the new (and better) marked plugin api instead of maintaining it ourselves. New renderer is hooked up and restores functionality to what it was before. 

With this change, I had to switch the way menus are built. All the pages markedown is now rendered before we do anything else, and the menu is built dynamically as part of that process. That likely means that issue #13 which I had a fix pending for will need some more thought. Another minor change is that IDs are now unique throughout the documentation. Should be minor as it would only effect duplicated IDs from different pages, which was broken in the menu anyway.