Adding fullscreen of editor area in playground

This adds a button to toggle fullscreen mode on the playground without the header and footer and introductory text.

Here is a [demo](http://bollwyvl.github.io/json-ld.org/playground-fullscreen/playground/).

This approach gets around some of the limitations of the outer responsive bootstrap `container` by only fullscreening a second, interior `container-fluid`. This does make the "flash of unstyled textareas" issue more pronounced (expecially on a slow connection), so it could be that needs to be solved first.