Support for compose files with different names (as per spec)

[mstrhakr]

PR from HERE

Requested HERE

Fixes #31

This would be correct behavior according to the docs:

The default path for a Compose file is compose.yaml (preferred) or compose.yml that is placed in the working directory. Compose also supports docker-compose.yaml and docker-compose.yml for backwards compatibility of earlier versions. If both files exist, Compose prefers the canonical compose.yaml.