From #1694.
a hook so plugins can implement their own search engine detection as there are requests for sites to be added that don't quite fit the traditional definition of a search engine.
(In [3421]) refs #1877 - add hook
(In [3422]) refs #1877 - pass referrer to plugin
ToDo: changes to plugins/Referers/functions.php to support custom search engines, e.g., path to image, search engine name to URL, etc.
In 2e215ff8752cef2ce80b43bd577cd5d322002abc: refs #1877 added hooks to add search engines and social networks; some code improvements