This will add functionality to automatically load ES plugins

This will setup and load any ES plugins and parses the configuration options for it
It also adds the client ( node, transport ) options to the configuration hash
Merge pull request #3 from logstash-plugins/feature/support_es_plugins

This will add functionality to automatically load ES plugins
Update Elasticsearch Output Plugin to retry bulk

Some actions may fail within the ES client bulk call.
Now, some messages (specifically errors 429 and 503s) will be
retried up to 3 times. If there are still actions that are
unsuccessfully indexed, Stud Buffer will continue its current
behavior (to retry indefinitely).

Stud Buffer will replay all events it first attempted to flush
to Elasticsearch. This means duplicate events may find themselves
in Elasticsearch.