closes #1980

* add clusterstats to elasticsearch input plugin

* add clusterstats to elasticsearch input plugin

* add clusterstats to elasticsearch input plugin

* add clusterstats to elasticsearch input plugin

* add clusterstats to elasticsearch input plugin

* responses to requested changes

* remove unnecessary recommendation

closes #1674

* Allow for TLS connections to ElasticSearch

Extremely similar implementation to the HTTP JSON module's
implementation of the same code.

* Changelog update

This constitutes a large change in how we will parse different data
formats going forward (for the plugins that support it)

This is working off @henrypfhu's changes.

This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.

closes #564

fixes #464

closes #430
closes #452