Provide ability to hook _add_scoring or option to use default search module scoring

Issue Summary

I've spent quite a bit of time trying to figure out why modifications to weights in Site Configuration -> Search Settings have had no impact on the quality of search results that the Faceted Search module returns. I found a few references to people suggesting that modifying these factors would impact search results.

As far as I can tell, modifying core search weights--including the installation of additional modules like search_ranking that provide additional features--does not have any impact on how faceted search scores search results.

This is because faceted search has it's own scoring mechanism based on node_search in faceted_search.inc called _add_scoring.

In my ideal world, I'd love for the search score to be derived from the settings set by core search or by modules that extend core search (e.g., search_ranking). However, absent that, I'd like to be able to hook add_scoring so I could override the scoring mechanism to add elements like statistics or reduce the impact of recency on results.

This is somewhat related to this issue #461978, but I'm not looking for a UI option to turn off scoring, but instead ways to get the scoring to behave more like the default search results.