init() {
    // Bind click events to each gallery
    getElementsFromOption(this.options.gallery, this.options.gallerySelector).forEach(galleryElement => {
      galleryElement.addEventListener('click', this.onThumbnailsClick, false);
    });
  }
Initialize lightbox, should be called only once.
It's not included in the main constructor, so you may bind events before it.