Add preview to workflow runtime metadata

[kelly-sovacool]

New feature

The workflow object has many useful properties, such as stubRun to indicate whether the workflow was invoked with the -stub/-stub-run CLI option. Similarly, it would be useful to access whether -preview was used.

Usage scenario

This is especially useful in the onComplete handler, e.g. to only send an email notification for "real" pipeline runs.

workflow.onComplete {
    if (!workflow.stubRun && !workflow.preview) {
        sendMail(...)
    }
}

Suggest implementation

Similar logic could be re-used from the other metadata that's populated from the CLI.

[greenberga]

If it's okay, I'll take a crack at this feature!