Once #7 is complete, implement basic support for the Elasticsearch output.

The simplest solution is to use the existing beats Elasticsearch output, but we should evaluate the effort required to switch to the official go-elasticsearch client (#14) with the initial output. If the effort is too high then switching the underlying Elasticsearch client can be deferred into a future iteration.

This feature is considered complete when at least the following criteria are satisfied:

• A test exists proving that an Elasticsearch output can be configured, and data published to the shipper is written to Elasticsearch.