Support stratified kfold

[vopani]

It would be very useful to support Stratified KFold which is way more commonly used than a random KFold.

Typically it'd look something like:

import datatable as dt
from datatable.models import kfold_stratified

X = dt.Frame(...)

stratified_splits = kfold_stratified(by=X['group'], nsplits=5, seed=1234)

where X is a frame having a column group by which the stratification is done.