get(unit) {
    return this[Duration.normalizeUnit(unit)];
  }
Get the value of unit.
@param {string} unit - a unit such as 'minute' or 'day'
@example Duration.fromObject({years: 2, days: 3}).get('years') //=> 2
@example Duration.fromObject({years: 2, days: 3}).get('months') //=> 0
@example Duration.fromObject({years: 2, days: 3}).get('days') //=> 3
@return {number}