add a new command minikube service foo

[jstrachan]

while in the docs this is handy:

kubectl get service $SERVICE --output='jsonpath="{.spec.ports[0].NodePort}"'

to get the nodePort. Then to append it to the output of minikube ip to be able to create a usable URL, it'd be nicer if there was a simple command to find the URL of a service on minikube.

So the user could type something like:

$ minikube service foo
http:///192.168.99.100:12345

So that folks can easily open HTTP/HTTPS services running inside minikube on their host operating systems without too much copy/pasting from the README to the command line ;)