fig local_port

Include one extra pr (https://github.com/docker/fig/pull/396), which will hopefully be merged soon-ish.

Adds a new fig command: `fig local_port` for printing the local port binding.
We can also use this from python code:

```python
def get_service_port(service_name, port, number=0):
    cmd = command.Command()
    project = cmd.get_project(cmd.get_config_path())
    service = project.get_service(service_name)
    return service.get_container(number=number).get_local_port(port)
```
This is actually a function which mutates internal state, so not a good candidate for a property (I think semantically `@property` implies it is not changing state).

I had considered making this private, but it's used as a public interface from the test suite, and I try not to go messing with interfaces too much so increase the changes that these changes will be merged into the upstream project.