helper.php: $opts as array for search()

#What: changing $opts parameter for search() from '' to an empty array
#Why: DokuWiki errors "Illegal string offset" for 'skipacl' / 'hash' in /inc/search.php
#Note: to test ...
Merge pull request #26 from hh-lohmann/tmp-fix-search.php-warnings

helper.php: $opts as array for search()