adding :visible and :hidden to zepto

gistfile1.js

;(function($){
  var _is = $.fn.is, _filter = $.fn.filter;

  function visible(elem){
    elem = $(elem);
    return !!(elem.width() || elem.height()) && elem.css("display") !== "none";
  }

  $.fn.is = function(sel){
    if(sel === ":visible"){
      return visible(this);
    }
    if(sel === ":hidden"){
      return !visible(this);   
    }
    return _is.call(this, sel);
  }

  $.fn.filter = function(sel){
    if(sel === ":visible"){
      return $([].filter.call(this, visible));
    }
    if(sel === ":hidden"){
      return $([].filter.call(this, function(elem){
        return !visible(elem);
      }));
    }
    return _filter.call(this, sel);
  }
})(Zepto);