This is to allow creating backups of the private key in case the YubiKey
gets lost or damaged. The word "insecurely" is appended to the name of
the switch and a long warning produced in order to make people who don't
know what they're doing turn away and stop what they're doing.

This is possible since piv-go 1.7.0[1] so I allowed myself to bump the
version of this requirement.

While I do have several doubts about approaching this (see below) I believe this
is a useful thing to have. What I'm unsure about is:

• The code quality (I'm not writing Go day to day)
• I may have done something stupid cryptography-wise here, I'm not a cryptographer or a security specialist
• How to warn properly so that people not knowing what they're doing don't use the new switch – I think I've done a decent job here
• Should this be mentioned in the readme? I mentioned it for documentation purposes
• Should this be mentioned in the --help output? The existing --really-delete-all-piv-keys flag is not there so I skipped it
• Should there be advice that when this option is used it's better to do it on a clean system, on a trusted machine, offline and airgapped ideally, and any potential backups should be encrypted and stored securely?
• Should the main agent daemon always handle keys that fail attestation (caused by generating on the computer instead of on the device) like I added here or maybe make this configurable with a switch?

Closes GH-65.

[1] go-piv/piv-go#83 (comment)