Make search mechanism pluggable

Currently, TinyTinyRSS can use raw SQL or the Sphinx search engine
for searching.  It would be nice if other search engines (such as
Xapian) could be used, or if features of the underlying SQL engine
(such as MySQL's FULLTEXT indexes) could be leveraged.  This commit
makes searching into a plugin hook, falling back to the builtin behavior
if no search plugin is active.  The Sphinx search behavior has been
broken out into a plugin.
Stop after first search plugin

The sanity check should have caught any extra ones, but it doesn't
hurt to be careful
mention config.php settings in search_sphinx plugin description
search_sphinx: abort if system client library exists