Add moving window cross-validation

[pplonski]

It can be useful in case of data with time values.

Please take a look at https://scikit-learn.org/stable/modules/generated/sklearn.model_selection.TimeSeriesSplit.html#sklearn.model_selection.TimeSeriesSplit

In such validation, stacking should be disabled.

[pplonski]

It is possible to do with a custom validation strategy. Fixed with #380.