Add an "init" option to copy in a custom docker-compose.override.yml file

As of #183, the supported method for overriding or extending the configuration of the underlying Docker containers is via a `docker-compose.override.yml` file.

As a convenience, it would be nice to have an option to `ff init` which allows pointing to an existing file that will be copied in as `docker-compose.override.yml` instead of the default empty stub file. This would allow reusing the same overrides multiple times without manual copy steps, such as if you delete and recreate a stack.