caddy secrets

This is a direct copy of the code that was originally hosted at https://github.com/startsmartlabs/caddy-secrets. To the people who wrote this originally, I can not longer build Caddy server from source because of this dependency which is required by freman/caddy-reauth/backends/refresh/auth.go. If you feel that you need more attribution (I cannot see the individual members of smartsmartlabs), or protest to have caddy-secrets be archived in this way, please contact me.

Special thanks for Andrew Lee for archiving this for me originally

I've decided to embed this so I can keep it up to date easily without the rather small external dependancy.

Go Proverb: A little copying is better than a little dependency.