Support --var and --vars-file flags for push.

[jmcarp]

I was going to submit a patch to handle https://github.com/concourse/cf-resource/issues/55, but it looks like we'll have to add interpolation-related flags here first. Also supports a use case described in #51.

cc @vito @jama-pivotal

[jmcarp]

@vito @xoebus @jama-pivotal: do you all maintain this project? It's used by https://github.com/concourse/cf-resource, which is owned by concourse, so I'm guessing you're the right people to tag, but lmk if not.

[drnic]

@vito @xoebus @jama-pivotal any feedback/changes for this PR?

[xoebus]

Thanks!