conf
Package conf is an extensible solution for cascading configuration. Package conf provides configuration processor that can load configuration layers from different sources and merges them into the one configuration tree. In addition configuration processor can expand references on configuration parameters in string values and process _ref and _include directives in resulting configuration tree. Package conf comes with built-in configuration loaders: fileconf and envconf, and can be extended by third-party configuration loaders. Package conf do not watch for configuration changes, but you can implement this feature in the custom configuration loader. You can find full example in repository.
See full documentation on GoDoc for more information.