Implement `extends`

[emk]

At the moment, cage really doesn't have any particularly good support for extends.

We could support this as follows:

extends:
  file: "templates/webapp.yml"
  service: "webapp"

Optionally, we could also have support for overriding templates:

• pods/templates/webapp.yml
• pods/targets/$TARGET/templates/webapp.yml

The trick would be to merge these two files before we use them as input to the extends. This would allow us to handle cases where we use one image in development, and a different image in production.

[seamusabshere]

heh, no more extends in latest version of docker-compose.yml :)