Add filter index name.

Useful for indices that are composed of multiple ES indices. Eg 10mil blogs in one index, and 10mil in another.

Certain operations (eg GET or search within a blog) require directing the search at a particular index rather than at a wildcard
Add document iterators to wpes-lib doc builders

Iterators help aid in bulk indexing documents. Some indices may index all published posts, while others index posts regardless of post_status. Iterators cleanly tie the bulk indexing to the index builder and allow more generic bulk indexing code.
Update README; Add example code for how to use wpes-lib.

The WP.com es_api_*() calls are too specific to WP.com to be used directly, but they make good example code for how to use wpes-lib.