convert a keyword search into a faceted term search

* provided we can convert the search keyword into an authoritative facet term
* if more than one facet is matched, we'll use the first one with the highest priority configured

related to these pull request: https://github.com/GigaOM/gigaom/pull/5114 and https://github.com/misterbisson/scriblio-authority/pull/76

see https://github.com/GigaOM/legacy-pro/issues/3995
Can we do an array merge instead of foreach? This [appears to be an array](https://github.com/wluo/scriblio/blob/github-3995/plugin/class-facet-taxonomy.php#L50).