count(totalLength, pageLength) {
    let spreads, pages;

    if (this.name === "pre-paginated") {
      spreads = 1;
      pages = 1;
    } else if (this._flow === "paginated") {
      pageLength = pageLength || this.delta;
      spreads = Math.ceil(totalLength / pageLength);
      pages = spreads * this.divisor;
    } else {
      // scrolled
      pageLength = pageLength || this.height;
      spreads = Math.ceil(totalLength / pageLength);
      pages = spreads;
    }

    return {
      spreads,
      pages
    };
  }
Count number of pages
@param  {number} totalLength
@param  {number} pageLength
@return {{spreads: Number, pages: Number}}