Add Sablier Traefik plugin.

[mcmikemn]

Sablier may or may not provide beneficial functionality, and may or may not be a good fit for d.ry. I'm trying to add it in order to check it out.

If we add it, this would implement #400

[mcmikemn]

This project turned out to be beyond my skillset. It requires:

• adding a Traefik plugin
• possibly needing to template a docker-compose.override.yaml file for traefik installation (see here)
• possibly defining Docker labels for apps that use Sablier differently than d.ry currently does (see here)

My commits so far are purely guesswork. I haven't even tested it yet, but I'm sure it will need more than I understand in order to work.

I'm going to shelve this project. If you think it's worth looking into, @EnigmaCurry, I'd love to work on it with you (even if that means that I just watch you and ask questions). If you're not interested in it, we should eventually just close this draft PR and the corresponding issue.