log something when purchasing an app

[z-tux-mind]

Description

First of all thank you very much for this awesome and handy tool, and for maintaining it as well!

It would be great to be able to tell if the app was purchased by ipatool or not when using the download --purchase command.
I guess that a simple debug log could go in download.go or in appstore_purchase.go.

In this way I can understand if the app was purchased by ipatool wihtout having to run it twice ( purchase and then download ).
Thank you so much for your help and precious time.