Gordon CLI

*I thought I'd start a long-running code review branch to track the progress of completing this feature and open the branch up to feedback before it's done.*

This branch aims to create a CLI for interacting with Warden containers. It is a Go replacement of the original `warden-client`. However, where `warden-client` was a REPL this CLI will just allow one command per execution in order to keep it simple. Initially the output will be presented in a human-friendly format but by the end of the branch it should be possible to use the CLI within scripts to orchestrate the containers.