Note: this is partially duplicating the template_from_string functionality:
http://twig.sensiolabs.org/doc/functions/template_from_string.html
We can't use that functionality though, since it requires the Twig_Extension_StringLoader()
extension. If we would use that, when instantiating Twig, it screws up the rendering: Every
template that has a filename that doesn't exist will be rendered as literal string. This
_really_ messes up the 'cascading rendering' of our theme templates.