Improve class extensibility, add filters to elastic search arguments and word count

* Change 'private' methods and variables to 'protected' to make extending the class easier.
* Add a filter for the arguments passed to es_api_search_index to make it possible to override keywords.
* Add a filter for $word_count in get_keywords() to make it easier to use a larger word count without extending the class. Enforce a (sane?) min/max word count.