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 think using the existing naming scheme will be enough to avoid merge conflicts, yes.

I think the decoupled image name you're talking about is the same as the tags from #457? 

I think specifying both `build` and `image` maybe be confusing, because you'd be re-using image to mean two things (either source image or destination image) based on the presence of another field.  Right now specifying both raises an error.