Make document structure configurable

Allow 3 options for document structure:

1) Elasticsearch document is identical to Couchbase document

2) Elasticsearch document is like Couchbase document, but with an additional top-level field containing Couchbase metadata (cas, seqno, vbucket uuid, etc). Metadata field name to be configurable, default to something like "$cbmeta".

3) Elasticsearch document has two top-level fields, "doc" (document content) and "meta" (couchbase metadata).

Option #1 is probably a good default. It performs the best (document bytes can be handed directly to Elasticsearch) and at least some applications won't need the metadata.

Option #2 has been requested by users. See  Github issue #113. Q: will those users be satisfied by option #1, or do they really need the metadata?

Option #3 is important because this is how the old connector wrote the documents, and it would simplify migrating applications to use the new connector.