Consider supporting SOP tools for PGP

[mdosch]

Currently profanity depends on gnupg for PGP (legacy PGP and OX). It would be nice to also allow the user to use a SOP backend. This would maybe also speed up OX in profanity. While encrypting/decrypting OX messages in go-sendxmpp happens almost instantly there is a noticable delay in profanity. As gosop uses the same PGP library as go-sendxmpp this might also improve the UX when using OX in profanity. And in general it may be beneficable to support more PGP backends and not hard depend on Gnupg in the possibly upcoming PGP schism.