Support YAML environment file

[austinkelleher]

It would be great if more complex environment variable types could be easily expressed without needing to serialize values as strings (e.g. arrays of values).

We should consider supporting .env.yaml at the root of the project and continue to support backwards compatibility with .env. If both a .env and .env.yaml are present, we should exit the program and signal to the user that only one of the supported files should be present. Additionally, .env.yaml should become the default configuration file name. We'd want to update the integration-template project once we have this new feature supported.

Relevant file extensions: .yaml, .yml