Add auth + all other ElasticSearch options including URI hosts support

This PR builds on [the auth PR](https://github.com/uken/fluent-plugin-elasticsearch/pull/56) of @farcaller, but completes the functionality required to support all possible ES connection options.  You will also notice I abandoned the hosts support using three consecutive colons to delimit the fields as I do not think it's a very scalable solution, so have instead opted for the use of a `URI`.

Specifically this PR provides support for:
* username & password
* path
* scheme
* list of full URIs within the hosts option such as http://john:password@logs:9200/es/

There is also test coverage.