Backup support

[je-sendra]

It would be really cool if you could export all your passes (or a single one) to back them up.

This should ideally work with .pkpasses as seen in #503 so this would be a dependency.

Edit: After further research on the subject, I have seen ".espass" is used when sharing a pass (looks like a new format). Since these ".espass" contain extra metadata not present in a ".pkpass", I guess it should export a zip file with all the ".espass" files. Trying to back them up manually by sharing, downloading and renaming the passes right now seems like a tedious process.