Fig port command

This branch adds a new fig command, which returns the local port binding of a service.

It's used when you want to run a test suite (or something else) outside of a docker image, so you can connect to one of your containers easily.

I will be using it mostly programatically, but I thought I'd add a command at the same time.
Is there a reason we're not just using `self.containers()` here, instead of manually filtering?