cli: Enable multiple environments in one command

[AaronFriel]

When a Pulumi stack has multiple environments (see: #193), to create a shell with those same environments in the CLI requires multiple invocations:

Given a stack with:

environments: [foo, bar]

To run a command in the same environment requires running:

pulumi env run foo -- pulumi env run bar -- [command]

We should support opening multiple environments in sequence just as Pulumi IaC does, for example, supporting a comma-delimited list of environments:

pulumi env run foo,bar -- [command]