Add a way to include cumulative running average with histogram aggregation results

[dakrone]

With something like this:

I can mentally draw a line representing the cumulative average through the histogram (because it follows a general pattern), but it would be nice if we could return the cumulative average of each bucket and its preceding buckets so Kibana could draw that line overlaid on the histogram.

This would be especially helpful for widely varying histograms, like:

[magno32]

This would be awesome...

[clintongormley]

Closed in favour of #8110