[Feature] add shipment selection in admin

[mokabiwd]

First of all, thank you for this module. As I have to rework a lot of the original mails, this is a very valuable module. However, there is a missing feature in the administration: the ability to select a shipment. Without it, it's impossible to display tracking numbers in the preview and the sending of mails:

• New Shipment
• New Shipment for guest

[jissereitsma]

Thanks. The module actually assumes that an order is there. Did you make sure that the order is created with a shipment and an invoice? https://github.com/yireo/Yireo_EmailTester2/blob/master/Model/Mailer/Variable/Shipment.php#L96