allow for limit facets by constrains but not current one

I add new pull request (prev https://github.com/ankane/searchkick/pull/84) with packed and rewrite code. Now its much cleaner and better with better (more complex test). I hope you like it and merge. 

I change a little your code (do not delete from options values for geo stuff) just skip them later. With that it was a lot easier to access original options and use it in facets. Hope you don't mind ;]