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)
```
Will all the ports be returned as strings or ints? I don't really know but this issue has bothered me from time to time so just making sure it's thought about.