Support sub aggregations

[dispalt]

I am looking to "nest" aggregations as described in the Structuring Aggregations page. Basically the usecase is to create a drilldown on a particular aggregation. An example would be if you are doing an aggregation to get the countries used in your application, and then you want to create a countries and avg latency breakdown, you would use sub aggregations.

[dispalt]

Never mind, it exists, https://github.com/sksamuel/elastic4s/blob/28b7ea65612495d722766c7c9ce0e35199747c4f/elastic4s-core/src/main/scala/com/sksamuel/elastic4s/aggregations.scala#L46