code: function placeholderUrl(width, height, text) {
      var url = 'http://placehold.it/' + width;
      if (height) {
          url += 'x' + height;
      }
      if (text) {
          url += '&text=' + encodeURIComponent(text);
      }
      return url;
  }
docs: get placeholder image url.
  
  @param {number} width
  @param {number} [height=width]
  @param {string} [text]
  @returns {string} placeholder image url