Add pulumi/pulumi_stack select

[chrsmith]

We should add a pulumi/stack_select orb. It isn't needed in many situations, because you can add a --stack flag to the up or preview commands.

However, when using Pulumi in CI and creating ephemeral environments you need to "select a stack, or create it if it is the first time referencing it". (https://github.com/pulumi/circleci/pull/30 describes this type of situation.)

By having a way to express pulumi stack select via an orb, it would be easier to use Pulumi in CircleCI.

[rarous]

With Pulumi 3.0 it is actually needed, because --stack behaves differently.