Add 'generate' command that outputs docker compose v3 file from project definition

Note: be aware that #7 will be changing how commands are added.

Add a new 'generate' command. It is expected that it will be supplied with an already parsed 'Project' definition. It can make an intelligent assumption about what the name of the image for each component will be. The logic that contains this assumption should be encapsulated in one place and easy to change.