Add "fig push" command

This command pushes any docker images that are declared as builds in the fig.yml to a docker repository. You can figure the repository using --repository-name or -r at the command line. This is a good starting point for figuring out how to deploy a fig stack into a production/cluster environment without having to make any assumptions about what this environment looks like.
Yes, I was thinking that the tags field from #457 would be that extra field. That way `docker push` could push all of the tags.