ENH: rolling()  for discontinuous time series data
Here is a discontinued time series data
|DATE|VALUE|
|----|----|
|2020-01-01|1|
|2020-01-02|1|
|2020-01-05|1|
|2020-01-06|1|


when I use rolling function

`
df.rolling(2, on='A', min_periods=2).sum()
`
OUTPUT is
|DATE|VALUE|
|----|----|
|2020-01-01|NaN|
|2020-01-02|2|
|2020-01-05|2|
|2020-01-06|2|


However, I expect the result as follows：
|DATE|VALUE|
|----|----|
|2020-01-01|NaN|
|2020-01-02|2|
|2020-01-05|NaN|
|2020-01-06|2|

It would be appreciated that the feature can be add in next update.


