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)
```
It doesn't need to be, but it's pretty meaningless outside of this function.

My guideline is usually if the function is just a helper (something that might be private if it were not a nested function), I make it nested. If for some reason it's complex enough that I need to test it on it's own, or can be re-used in some other place,  I'll break it out.