code: function setup(options) {
          const config = Object.assign({}, DEFAULT_CONFIG, options);
  
          const $slider = config.$el;
          const $track = $slider.querySelector('.slider-track');
          const $after = $slider.querySelector('.slider-after-image');
  
          // Copy styles.
          $slider.style.width = getComputedStyle($after).getPropertyValue('width');
          $slider.style.height = getComputedStyle($after).getPropertyValue('height');
  
          $track.addEventListener('input', (evt) => {
              const percent = Number(evt.target.value);
              $after.style.width = `${percent}%`;
          });
  
          $track.value = config.defaultValue;
          $track.dispatchEvent(new Event('input'));
      }
docs: Function which initialize before/after slider.
  
  @param {Object} options - Default settings