Add support for nested aggregations

[barseghyanartur]

Add support for nested aggregations.

Some references:

• https://www.elastic.co/guide/en/elasticsearch/reference/5.5/search-aggregations-bucket-nested-aggregation.html
• https://github.com/elastic/elasticsearch/issues/11749
• https://github.com/elastic/elasticsearch/issues/12410

Update 2018-06-27

This is on-going issue. At the moment, there's a temporary solution for that (by making a custom filtering backend). See the documentation and examples:

• custom nested aggregations backend
• docs

Update 2018-08-02

https://github.com/elastic/elasticsearch-dsl-py/blob/master/elasticsearch_dsl/faceted_search.py#L161

elasticsearch-dsl version 6.x supports nested aggregations, however, since this library aims to support 2.x (not so important anymore, might as well be deprecated) and 5.x, the total switch to 6.x functionality can't be made (simple), unless we copy some of the elasticsearch-dsl functionality locally or have it supported for Elasticsearch 6.x only. Help here is appreciated. If you need this badly, either see my remarks above (2018-06-27, regarding custom nested aggregations backend) or consider making a contribution (I will assist you).