Add ingest pipeline support

[audriusbugas]

Working on cabana becomes very hard once we start indexing contexts or instruct events. Field names simply gets very large. e.g.

unstruct_event_com_hometogo_event_conversion_2.category

As a result it's hard to use cabana dropdown, etc. The solution could be allow modifying field names via elasticsearch ingest pipelines https://www.elastic.co/guide/en/elasticsearch/reference/master/pipeline.html . This just one additional flag when indexing document, must be very easy to implement and should bring great value for using large scale with bigger teams.