added use_keys argument to array_filter()

This adds a third (optional) argument to `array_filter()` that will send the array key as the second argument to the given callback function.

````
    function myfilter($value, $key)
    {
        return $key > 4;
    }

    array_filter(array(1, 2, 3, 4, 5, 6, 7), 'myfilter', true);
    // returns: [6, 7]
```