Add support of valid 'enclosure' element

[gulch]

Added support of 'enclosure' element with attributes.
Example:
<enclosure url="https://funtime.kiev.ua/uploads/img/grid/middle/2016/03/holod-do-drozhi-vzaperti-kiev-kvest-komnata-56f95fdecc6c2.jpg" type="image/jpeg" length="74061"/>

Code example:

$feed = new \Thujohn\Rss\Rss();
$feed->feed('2.0', 'UTF-8');
....
$item['enclosure'] = [
    'url'=> $image_url,
    'type' => mime_content_type($image_path),
    'length' => filesize($image_path)
];
$feed->item($item);