Expose config option to decrypt unauthenticated messages

[julianosk]

As other PGP libraries, the option to allow the decryption of unauthenticated messages should be exposed from the crypto.KeyRing.Decrypt* methods.

There is already an option on the go-crypto/openpgp/packet/Config struct: InsecureAllowUnauthenticatedMessages, but there is no way of setting it to true.