[Preliminary] enable ssh support if set in gpg-agent.conf

This module is now able to detect if ssh-support is enabled in
*~/.gnupg/gpg-agent.conf*. Based on the check in ssh agent
(currently developed in branch module/ssh), it checks whether
any identities have been loaded yet and if not, does that. It
also loads multiple identities if so configured in *zpreztorc*.
*README* updated accordingly.
Merge branch 'module/gpg'