Allow restructuredText as render markup 

I've researched a while looking for a simple yet usable wiki engine with support for restructuredtext, due it's what I write fluently and the I can use the content to render with Sphinx (de facto standard documentation tool in the python ecosystem), rst2pdf or whatever
Few wikis like MoinMoin or the builtin in Trac has this feature, but by processing a "block of restructuredtext" in the middle of core markup. 

Even when this project uses markdown, it's so simple to refactor this "render function" as a config option. 

I've just done a dirty proof of concept in my fork.