New hooks

Issue Summary

This module is excellent out of the box, but becomes limited when you need your own document attributes and additional filtering, you noted in you readme the following to do:- provide new hooks to allow external modules extend Sphinx document attributes and/or alter search user interface with additional filters, please can you consider adding this as a high priority.

You definately need to be able to affect the following $sphinxsearch->SetFilter, $sphinxsearch->SetSortMode and $sphinxsearch->SetGroupBy (there are probably others) currently in sphinxsearch.common.inc. Would also be useful to change some of the static text and make it configurable e.g. "Search service is disabled temporarily. Please, try again later."