ENH: Add 'end' option in resample's origin
In some cases I need to calculate the time group from the end of time series instead of the beginning.

For example, if user uses `s.resample('7min', origin='end', closed='right')` , the following expected groups are (09:23:45, 09:30:45], (09:16:45, 09:23:45], (09:09:45, 09:16:45], ... , (08:20:45, 08:27:45]. And under normal cases, `closed` should be `right` when `origin` is `end` since last value should serve as the end value of the last group.

```python
>>>idx = pd.date_range('20200101 8:26:35', '20200101 9:31:58', freq='77s')
>>>idx
DatetimeIndex(['2020-01-01 08:26:35', '2020-01-01 08:27:52',
               '2020-01-01 08:29:09', '2020-01-01 08:30:26',
               '2020-01-01 08:31:43', '2020-01-01 08:33:00',
               ... ...
               '2020-01-01 09:25:37', '2020-01-01 09:26:54',
               '2020-01-01 09:28:11', '2020-01-01 09:29:28',
               '2020-01-01 09:30:45'],
              dtype='datetime64[ns]', freq='77S')
>>>data = np.ones(len(idx))
>>>s = pd.Series(data,index=idx)
>>>s
2020-01-01 08:26:35    1.0
2020-01-01 08:27:52    1.0
2020-01-01 08:29:09    1.0
.... ...
2020-01-01 09:23:03    1.0
2020-01-01 09:24:20    1.0
2020-01-01 09:25:37    1.0
2020-01-01 09:26:54    1.0
2020-01-01 09:28:11    1.0
2020-01-01 09:29:28    1.0
2020-01-01 09:30:45    1.0
Freq: 77S, dtype: float64
```

Thus the sum operation on this resampler shows:

```python
>>>s.resample('7min', origin='end', closed='right').sum()
2020-01-01 08:27:45    1.0
2020-01-01 08:34:45    6.0
2020-01-01 08:41:45    5.0
2020-01-01 08:48:45    6.0
2020-01-01 08:55:45    5.0
2020-01-01 09:02:45    6.0
2020-01-01 09:09:45    5.0
2020-01-01 09:16:45    6.0
2020-01-01 09:23:45    5.0
2020-01-01 09:30:45    6.0
Freq: 7T, dtype: float64
```