lock body scroll when showing overlay

[pgherveou]

I hard coded something like that in my local branch, to lock the scrolling when showing the ovelay
Do you think this is an option we should expose to the api ?

  Overlay.prototype.show = function(){
    $('body').css('overflow', 'hidden');
   ...
  };

  Overlay.prototype.hide = function(){
    $('body').css('overflow', '');
   ...
  };