Don't create sidecar if there is no x-mutagen section

[xwillq]

Mutagen Compose version: 0.15.0

If docker-compose.yml doesn't have x-mutagen section, sidecar container won't do anything useful, it will just occupy space in docker ps output. So, maybe it would be better to not start sidecar container in this case?

And to extend this, other features, like extended output of mutagen-compose ps, could be disabled too, so mutagen-compose could be a drop-in replacement for docker-compose in projects, where it isn't required.