Enable multiple values

[leodido]

The name and native fields can contains multiple values.
See here, and here for example.

Ensure that this works.

$adapter = new LanguageCode();
$options = new LanguageCodeOptions();
$converter = new Conversion(['adapter' => $adapter, 'options' => $options]);
$options->setOutput('iso639-1');
// NATIVE => ISO 639-1
$converter->filter('valencian'); // ca

End user should not be forced to do.

// ...
$converter->filter('catalan, valencian'); // ca

Handle comma separated values (or convert the array).