the date-picker component support to show Customize the initial month

[SubinY]

What problem does this feature solve?

At the bussiness scene, the nz-range-picker show the double month to select by the user to search their order which is past time. But the date comp have not support to change or edit the init month

What does the proposed API look like?

In nz-range-picker comp, the left panel show the last month or last year. we can add a Input property to realize this function

####date-range-popup.component.ts (line 394)

...
  @Input() dropdownClassName: string;
  @Input() showLastMonth: Boolean;
...
  private normalizeRangeValue(value: CandyDate[]): CandyDate[] {
    const [ start, end ] = value;
    const newStart = start || this.showLastMonth ? new CandyDate().subMonths(1) : new CandyDate();
    const newEnd = end && end.isSame(newStart, 'month') ? end.addMonths(1) : end || newStart.addMonths(1);
    return [ newStart, newEnd ];
  }

####candy-date.ts (add the api subMonths)

  subMonths(amount: number): CandyDate {
    return new CandyDate(subMonths(this.nativeDate, amount));
  }

It prototype look like this. I try to add a property to custom the init month or year in this comp, but i found is involve many comp to add this property

[tonny008]

如果选这range是用于历史记录的，那这个还是很需要的。
如默认range当天，但是日历出现的是当月和下月，但是就使用场景应该是上月和当月比较合理