Added gpg-agent plugin

Based on ssh-agent plugin.

plugins/gpg-agent/gpg-agent.plugin.zsh

+# Based on ssh-agent code
+
+local GPG_ENV=$HOME/.gnupg/gpg-agent.env
+
+function start_agent {
+  /usr/bin/env gpg-agent --daemon --enable-ssh-support --write-env-file ${GPG_ENV} > /dev/null
+  chmod 600 ${GPG_ENV}
+  . ${GPG_ENV} > /dev/null
+}
+
+# Source GPG agent settings, if applicable
+if [ -f "${GPG_ENV}" ]; then
+  . ${GPG_ENV} > /dev/null
+  ps -ef | grep ${SSH_AGENT_PID} | grep gpg-agent > /dev/null || {
+    start_agent;
+  }
+else
+  start_agent;
+fi
+
+export GPG_AGENT_INFO
+export SSH_AUTH_SOCK
+export SSH_AGENT_PID
+
+GPG_TTY=$(tty)
+export GPG_TTY