Cumulative aggregation

[martsberger]

It might be useful to some people to have the aggregation be cumulative.

From issue #5 (PJCampi)
It would be fairly easy to allow for cumulative aggregation as well, which can be useful. It can be done using the window function (+distinct):

window = Window(aggregator(data), order_by=row)

This would be a feature or option in addition to the current aggregation.