Add extension hook to ShopEmailPreviewTask

[wernerkrauss]

ShopEmailPreviewTask is a cool feature to test all emails related to a shop.

It'd be great if I could add custom specific emails (e.g. invoice with pdf) for easy testing.

TODO:

• [ ] add extension hook
• [ ] document how to add other emails in an extension

[wernerkrauss]

fixed by #799