flag: add a Value example

Change-Id: I579cc9f4f8e5be5fd6447a99614797ab7bc53611
Reviewed-on: https://go-review.googlesource.com/120175
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/flag/example_value_test.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flag_test
+
+import (
+	"flag"
+	"fmt"
+	"net/url"
+)
+
+type URLValue struct {
+	URL *url.URL
+}
+
+func (v URLValue) String() string {
+	if v.URL != nil {
+		return v.URL.String()
+	}
+	return ""
+}
+
+func (v URLValue) Set(s string) error {
+	if u, err := url.Parse(s); err != nil {
+		return err
+	} else {
+		*v.URL = *u
+	}
+	return nil
+}
+
+var u = &url.URL{}
+
+func ExampleValue() {
+	fs := flag.NewFlagSet("ExampleValue", flag.ExitOnError)
+	fs.Var(&URLValue{u}, "url", "URL to parse")
+
+	fs.Parse([]string{"-url", "https://golang.org/pkg/flag/"})
+	fmt.Printf(`{scheme: %q, host: %q, path: %q}`, u.Scheme, u.Host, u.Path)
+
+	// Output:
+	// {scheme: "https", host: "golang.org", path: "/pkg/flag/"}
+}