This change adds a `pulumi stack output` command. When passed no
arguments, it prints all stack output properties, in exactly the
same format as `pulumi stack` does (just without all the other stuff).
More importantly, if you pass a specific output property, a la
`pulumi stack output clusterARN`, just that property will be printed,
in a scriptable-friendly manner. This will help us automate wiring
multiple layers of stacks together during deployments.
This fixespulumi/pulumi#659.
