envflag.go
func String(name string, value string, usage string) *string {
	return EnvironmentFlags.String(name, value, usage)
}
func String(name string, value string, usage string) *string {
	return EnvironmentFlags.String(name, value, usage)
}
// String defines a string flag with specified name, default value,
// and usage string.  The return value is the address of a string
// variable that stores the value of the flag.