Allow a more broader set of fields to be used as index & facet, plus allow indexing in other language

In an last ditch effort to get this plugin working for me i added the following patches to the code base. 

In short this changes the following things in the plugin
- It allows the user to change the analyzer language through the elasticsearch_string_language filter
- It allows the user to select "post" fields other then post_title, _context or _type (excerpt for example) for indexing through te elasticsearch_default_fields filter
- It allows the user to add custom fields other than the one found by Defaults::meta_fields (this was needed to index the meta field _regular_price in my woocommerce install, which wasn't found because it starts with a _), through the filter elasticsearch_default_meta_fields
- Make faceting on meta fields possible. They were currently not in the selected options (again needed to facet on price in woocommerce)

I added a unit test for the defaults filters i added. All other tests seem to work ok.