Bare links wrapped in [url] don't get converted

[ericek111]

Thanks for your library, it helped me a great deal.

However, this should either be converted to Markdown link ([https://github.com](https://github.com)) or the URL tags should be stripped:

$input = '[url]https://github.com[/url]';
$Converter = new \Converter\BBCodeConverter($input);
$output = $Converter->toMarkdown();
// $output == $input