Implement docker-compose.yml Format

[MeneDev]

Implement the dockfmt.Format interface for docker-compose.yml.

Design goal: When the file is changed, every change must be as local as possible. That means that white-spaces (including line-endings) and comments are retained. Only the image reference may change.

While this is not strictly blocked by #5 / #13, the design goal only makes sense afterwards.