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.
So to avoid confusion it sounds like it would be best to just have a separate field to track images want to be pushed. Maybe this command is not even needed. It basically reduces to a shortcut around "docker push" for a group of images that each play a role in the current fig file.