General purpose fire-and-forget command "wrapper" for Pipeline

Note - may move this out of the workflow-plugin component if/when I end up writing this as a separate plugin.

It'd be really nice to have the ability to fire a sh/bat command in the "background" and have it keep running until the build is done with it. Yeah, this can be done by hand now, but not very elegantly. So I'd like to add a wrapper specifically for Pipeline that would work like this - launching a given shell or batch command, running the body passed to the step in Pipeline, and then killing off the shell/batch command upon completion of the body.