Add an ability to Save plan files

[bheemvennapureddy]

It would be a nice to have to save the plan files when we run the plan command so that someone can verify the plan files rather than looking at the console log for the plan command and pass the plan files if needed to apply command so that we don't have to regenerate the plan.

[bheemvennapureddy]

@bplunkett-stripe as per discussion from #154, are you good with this?