[Feature] Support for displaying Pulumi stack

[scottslowe]

Is your feature request related to a problem? Please describe.
I use Pulumi for infrastructure-as-code use cases. Pulumi has the concept of multiple stacks (think of them as conceptually similar to Terraform workspaces). Aside from running pulumi stack ls, there's no easy/straightforward way to tell which stack is active.

Describe the solution you'd like
It would be ideal to have powerline-go display the currently-active Pulumi stack when in a directory where a Pulumi project exists. This would be similar to the way that powerline-go can display the active Terraform workspace.

Describe alternatives you've considered
I use powerline-go on all the systems where I code in/use Pulumi. I don't think there are any other alternatives out there to display the currently-selected Pulumi stack in the shell prompt (not that I've found, anyway).

Additional context
N/A

[scottslowe]

@justjanne I have a draft implementation that I can submit as a WIP PR, if you're interested.