ElasticSearch: support embedded JSON payload

There is already a "payload" format option for the ES output plugin that simply returns the heka message payload as the JSON document.  It would be nice to use the "clean" or "logstash_v0" formats with the option to embed the message payload as JSON (rather than being escaped).  This has the advantage of capturing both metadata *and* a structured JSON payload.  Embedding is activated with "embed_json = true" option in the ES output configuration (defaults to false).

I'm opening this pull request now to see if there is interest in such a feature and if so, feedback on the implementation.  I can add tests to this PR if worth merging.