Histogram aggregations in v1

This is part of #504 (closed).

I will split into two classes of aggregations: TermsAggregation (what we have now) and HistogramAggregation. Will map this to ES histogram.

Clients will be forced to provide a width (or min, max + size). All ES aggregations (except for Dates) are fixed interval in 6.x. In ES 7.x there is a variable width histogram. This would allow to have a histogram based on a given number of buckets. The histogram will not be even sized though. Don't know if this is a problem for plotly ... but I guess the histograms might be quite nice in uneven data.

But going for ES 7.x might be quite some work? Maybe I give it a spin.