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.