Filter JSON search customers query

Because the json_search_customer_name action is in the scope of $this, and there's no instance of it to allow remove_action() to play nicely with, it's really difficult to modify this query.  And, unfortunately, it's quite non-performant on large user tables.

One possible option would be to allow users to filter this query directly.  This PR reflects that approach.

For reference: https://gist.github.com/curtismchale/9470986, #5095