cage clean: Add a way to clean up containers, volumes, etc.

This should only clean up containers and volumes associated with a project, and it should ideally be defined on top of existing `docker clean` or `docker-compose clean` subcommands, if those exist.

In particular, we want to automate things like:

```
docker rm -f $(docker ps -qa)
docker system prune -f && docker volume rm $(docker volume ls -q)
```