Add Pulumi stack support

[jerome-probabl]

This PR add support for Pulumi Stack. It's really similar to Terraform workspace.

If you want to test it just use powerline-go with pulumi-stack module activate:

function _update_ps1() {
    PS1="$($GOPATH/bin/powerline-go -modules=venv,pulumi-stack,user,host,ssh,cwd,perms,git,jobs,exit -hostname-only-if-ssh -error $? -jobs $(jobs -p | wc -l))"

    # Uncomment the following line to automatically clear errors after showing
    # them once. This not only clears the error for powerline-go, but also for
    # everything else you run in that shell. Don't enable this if you're not
    # sure this is what you want.

    #set "?"
}

if [ "$TERM" != "linux" ] && [ -f "$GOPATH/bin/powerline-go" ]; then
    PROMPT_COMMAND="_update_ps1; $PROMPT_COMMAND"
fi

Warning pulumi stack --show-name is not really fast so it can slowdown your prompt display, sadly there isn't other alternative to find the current stack.