New plugin: php manual lookup

Example: `php urlencode`
/search.php returns search results, but DDG uses /manual-lookup.php, which redirects to the searched-for function manual page, if any.
Example: http://php.net/manual-lookup.php?pattern=urlencode
(Using search.php) http://php.net/results.php?q=urlencode&l=en&p=all

`run()` should use http://php.net/manual-lookup.php directly though; I was using DDG before I found what script they were using behing the scene.