Add option to return API response or download labels on checkout

[dahoo]

Thanks for your work! It works great for us apart from the fact that we'd like to store the tracking code.

I added a download_labels parameter to checkout! to decide whether you want the old behavior or return the response with additional information.

[PatrickFranken]

Had to figure out first how my initial code actually worked after years not working on it 😆

Could you give an example output of "data" (L46)? I guess it would make sense to explain other developers a bit which data is returned. Right now it's just a basic PDF, missing any way of misinterpretation but that's obviously different with an hash.
Maybe just add an example to the README which gives an idea about what to receive if download_labels is false.

Anyway, thanks for your effort!