code: function countFromTo(koyomi, from, to) {
    var blocks = koyomi.separate(from, to);
    if (!blocks) {
      return null;
    }
    var days = 0;
    days += blocks.days.reduce((x, d) => koyomi.isOpen(d) ? x + 1 : x, 0);
    days += blocks.months.reduce((x, d) => x + countMonth(koyomi, d), 0);
    days += blocks.years.reduce((x, d) => x + countYear(koyomi, d), 0);
    return days;
  }
docs: 二つの日付の間の営業日数を返します
  @param  {Object} koyomi
  @param  {DATE}   from
  @param  {DATE}   to
  @return {Number} days