Contributing output-elasticsearch_groom plugin

I would like to contribute an output plugin that I developed:

https://github.com/itzg/logstash-output-elasticsearch-groom

There are several scripts/solutions out there, most notably [Curator](https://github.com/elastic/curator); however, that one specifically is Python based and others are typically cron-driven. (Nothing wrong with Python :), it's just yet another system dependency.)

I preferred a solution that was more logstash-y and facilitates a self-regulating logstash ecosystem of time-based indices. I also wanted to define `index` templates "once" and use that to define both the creation and grooming specifications.

I have included inline documentation and RSpec tests. It is also [published to RubyGems](https://rubygems.org/gems/logstash-output-elasticsearch_groom). Please let me know if more pipeline configuration examples would help to clarify the intent and usage.