Allow calling of render instead of forcing on load.

Make it so you call strapdown.render() when you are ready instead of
forcing on load, increases the usability of this by a lot.

Sadly, marked suffers from this as well so for the time being I made a change to your version of the minified code to add marked to the global namespace, alternatively I could have removed the code from the self executing function and instead had it declared within the strapdown object.