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.
I can revert the naming scheme change to just use the existing fig style. Will this be enough to avoid conflicts with your PR? Instead, push could filter for services that define both a "build" AND "image" property. This would allow a more descriptive image since the image name here would be decoupled from the fig service name.  The repo could also be specified here. It would also make it possible to limit which build you actually care about pushing (since it would only push builds that also have an image property defined). Does this sound better? other suggestions?