code: function getModifiersForDay(day) {
    var modifiersObj = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : {};
  
    return Object.keys(modifiersObj).reduce(function (modifiers, modifierName) {
      var value = modifiersObj[modifierName];
      if (dayMatchesModifier(day, value)) {
        modifiers.push(modifierName);
      }
      return modifiers;
    }, []);
  }
docs: Return the modifiers matching the given day for the given
  object of modifiers.
  
  @export
  @param {Date} day
  @param {Object} [modifiersObj={}]
  @return {Array}