Add letsencrypt:ls command showing apps and expiry
Add letsencrypt:ls command showing apps and expiry

Ref. #18, Adds a new `letsencrypt:ls` command that will list all apps
that are currently secured by letsencrypt and the time their certificates
will expire, with the most recently-expiring certificates listed first.

Together with a3c09f, this will enable creating an automatic renewal
procedure.

sseemayer/dokku-letsencrypt,dokku/dokku-letsencrypt