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

[peussen]

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 _regularprice 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.

[parisholley]

Thanks for the pull (and tests!), I left a few comments on your commits. If you could make those tweaks, i'll get this pulled!

[peussen]

Concerning the filters in the Defaults file: I had to place them there, otherwise they won't show up in the admin content indexing screen, which eventually led to a lot of search failures because the index was not good enough. Setting it in config (which i tried), will not make the fields show up in the content_indexing screen at all.

[parisholley]

But what need is there to show it in the admin, if you are telling the system in code to index those fields?

[peussen]

Seems i'm not the only one who likes to show things in the admin as i saw another pull request doing the same ;)

I removed the filters for now. I figured out what went wrong, and it seems more something that has to do with the wipe data which creates a proper mapping, whereas rebuild index seems to lets elasticsearch figure it out..