This allows you to select and configure the service without
touching the main compose file, making updating a lot easier.

Also add overrides for MyDNSHost and HTTP.

Serve /.well-known/acme-challenge/ for http-01 challenge.

Issue csmith/docker-automatic-nginx-letsencrypt#3

Allow nginx-updater to see /letsencrypt

This is required for csmith/docker-service-nginx#3

The previous one didn't really work (the way we update files
doesn't trigger the modify event) and broke for newer versions
of Docker (which require a HTTP Host header).

Fixes #5

Closes #6

These are included within a http block, and it's illegal to nest them.

Define names for everything instead of using the default
compose names.