Add the positive keys to the $result array in search results.

This task stemmed from the feature request at #685758: Need a hook to allow alternative snippet creation for search results. The positive keys array should be added to the $results array in luceneapi_node_search() so that developers have access to it in hook_luceneapi_result_alter() implementations. This is a bug because developers cannot easily access the positive keys making it difficult to create custom snippets with highlighting.