code: func NewJSONSourceFromFlagFunc(flag string) func(c *cli.Context) (InputSourceContext, error) {
  	return func(context *cli.Context) (InputSourceContext, error) {
  		return NewJSONSourceFromFile(context.String(flag))
  	}
  }
docs: // NewJSONSourceFromFlagFunc returns a func that takes a cli.Context
  // and returns an InputSourceContext suitable for retrieving config
  // variables from a file containing JSON data with the file name defined
  // by the given flag.