[WIP] Delete all records matching a query

Sorry, I'm having a bit of trouble digging through the code.

I think that this basic structure is correct for adding this functionality, but I'm unsure what to write in the `update_delete_all` method. Should it update some flag in the `request_options` `hash_storage`?

After that, where is the code that actually sends the request to ES? Will need to have it check for the new delete flag and send a DELETE request if it is true.
Just delegate `delete_all` to index here: https://github.com/toptal/chewy/blob/master/lib/chewy/index/search.rb#L7