Add 'date_math_index_name' processor

[martijnvg]

We should add a processor that overrides / sets the index based on a timestamp / date field. This is useful for event ingestion where the actual date of an event is inside the source.

This processor should utilize the date math index name support:
https://www.elastic.co/guide/en/elasticsearch/reference/master/date-math-index-names.html

The processor should look something like this:

PUT /_ingest/pipeline/test
{
  "processors" : [
    {
      "date_math_index_name" : {
        "timestamp_field" : "timestamp",
        "optional_date_format: ....,
        "index_name_template" : "events-{{date}}" // 'date' is the formatted date string
      }
    }
  ]
}