router.go
func (r *Router) Group(path string, fn func(r *GroupRouter)) {
	gr := NewGroupRouter(r, path)
	fn(gr)
}
func (r *Router) Group(path string, fn func(r *GroupRouter)) {
	gr := NewGroupRouter(r, path)
	fn(gr)
}
// Group takes a path which typically a prefix for an endpoint
// It will call callback function with a group router which
// you can add handler for different request methods