Define acceptable file formats for file configuration

[jack-berg]

Complete outstanding TODO to define acceptable file format.

The OTEP called for JSON or YAML to be supported, with a preference for YAML.

[jack-berg]

YAML file format was defined in #3744. There's an open TODO for defining JSON format.

I'm personally in favor of only supporting YAML, removing the JSON TODO and closing this issue. Introducing JSON would come likely come with language stating that implementations should support one of JSON or YAML, but need not support both. This would be a bad user experience since we'd loose the ability to have configuration files portable across languages. Not good. Better to support only YAML, even if it means adds a dependency on a YAML parser.