encrypt keys using gpg secret key

[PACordonnier]

Keyser only supports symmetric KEYSER_GPG_MODE. We could use asymettric encryption. User should define a KEYSER_GPG_ID with its secret key id.

When encrytping, the public key will be used.
When decrypting the user will be prompted by gpg-agent to its secret key password and gpg will proceed with decryption

e.g: https://github.com/zx2c4/password-store

[wdavidw]

I left it as a future improvement.