use League\CommonMark\CommonMarkConverter;
use League\CommonMark\Environment;
use League\CommonMark\Ext\Strikethrough\StrikethroughExtension;

// Obtain a pre-configured Environment with all the CommonMark parsers/renderers ready-to-go
$environment = Environment::createCommonMarkEnvironment();

// Add this extension
$environment->addExtension(new StrikethroughExtension());

// Instantiate the converter engine and start converting some Markdown!
$converter = new CommonMarkConverter($config, $environment);
echo $converter->convertToHtml('This extension is ~~good~~ great!');