Touch Bar and Keychain support

Since name-constrained certs don't work everywhere, leaving the signing key lying around still exposes you to risk of having all of your secure traffic intercepted.

My first thought was that it would be nice to be able to keep the key on a Yubikey, but putting it in the macOS keychain under password/Touch ID protection (or something similar like GNOME keyring) would also be a reasonable intermediate option.

Do you think that would fit in the scope of this project, or should it perhaps be something separate?
(Is there already a PKCS#11 abstraction in Golang that would support this functionality?)