Added a filter for $query_string, to change query sent to swiftype.

Added in the `swiftype_search_query_string` filter, for users to manipulate the query before sending it to swiftype. This is similar to how `swiftype_search_params` works.

For Example:

```
function swiftype_search_query_filter( $query ) {
	list($keywords, $tags) = parse_tags($query);
    return $keywords;
}
add_filter( 'swiftype_search_query_string', 'swiftype_search_query_filter', 8, 1 );

```