static getSlideNumber() {
    const results = document.location.hash.match(slideRegex);
    let slide = 0;

    if (Array.isArray(results)) {
      slide = parseInt(results[1], 10);
    }

    if (typeof slide !== 'number' || slide < 0 || !Array.isArray(results)) {
      slide = null;
    } else {
      slide--; // Convert to 0 index
    }

    return slide;
  }
Gets the slide number from the hash by a regex matching `#slide=` and gets
the number after it. If the number is invalid or less than 0, it will
return null as an invalid value.
@return {?number}