code: func Environment(key string) *configSource {
  	configString := os.Getenv(key)
  	return &configSource{
  		skip: configString == "",
  		from: fmt.Sprintf("'%s' environment variable", key),
  		apply: func(baseConfig interface{}) error {
  			return FromString(configString, baseConfig)
  		},
  	}
  }
docs: // Source from a single environment variable with config embedded in JSON