FEAT: PGP signing/encrypting

[mdosch]

Is your feature request related to a problem?

I am missing PGP encryption.

Describe the solution you'd like

It would be awesome if you could add PGP:

• signing outgoing mails
• check signature of incoming mails
• encrypt outgoing mails
• decrypt incoming mails
• settings (e.g. sign all outgoing mails per default, encrypt mails per default if a key is present for a contact)

Maybe you could check github.com/emersion/go-pgpmail or github.com/protonmail/gopenpgp

Describe alternatives you've considered

No response

Additional context

No response

[andrinoff]

Yes, have been thinking about introducing PGP encryption since S/MIME introduction in #225.

This will take some time though.