Also, fix the postaggregation for approxHistogram quantiles so it adds
the dependent field and that can show up in the graphs/tables.

In general, postAggregators add significant power, we should probably
support including custom postAggregators. Plywood has standard
postAggregators here, and a customAggregator escape hatch that allows
you to define custom postAggregators.

This commit adds a similar capability for Superset and a additional
field/fields/fieldName breakdown of the typical naming for dependent
aggregations, which should make it significantly easier to develop
approxHistogram and custom postAggregation-required dashboards.

* break out CustomPostAggregator into separate class. This just cleans
  up the creation of the postaggregator a little bit.
* minor style issues.
* move the function around so the git diff is more readable