Make examples executable directly

Hi!

Thanks for writing and sharing this useful args parsing library!

As I'm still learning Go, it took me some time to make the provided examples work. I propose the apply in README.md the changes I had to make in order to make them work on my end, hoping that it would also help other potential users of your module.

Best regards,

Adrien

README.md

@@ -58,9 +58,13 @@ Documentation can be found at [Godoc](https://godoc.org/github.com/cosiner/flag)
 # Example
 ## Flags
 ```Go
-package flag
+package main
 
-import "fmt"
+import (
+	"fmt"
+
+	flag "github.com/cosiner/flag"
+)
 
 type Tar struct {
 	GZ          bool     `names:"-z, --gz" usage:"gzip format"`
@@ -73,7 +77,7 @@ type Tar struct {
 	SourceFiles []string `args:"true"`
 }
 
-func (t *Tar) Metadata() map[string]Flag {
+func (t *Tar) Metadata() map[string]flag.Flag {
 	const (
 		usage   = "tar is a tool for manipulate tape archives."
 		version = `
@@ -87,7 +91,7 @@ func (t *Tar) Metadata() map[string]Flag {
 		cpio, ar, and shar archives.
 		`
 	)
-	return map[string]Flag{
+	return map[string]flag.Flag{
 		"": {
 			Usage:   usage,
 			Version: version,
@@ -99,22 +103,21 @@ func (t *Tar) Metadata() map[string]Flag {
 	}
 }
 
-func ExampleFlagSet_ParseStruct() {
+func main() {
 	var tar Tar
 
-	NewFlagSet(Flag{}).ParseStruct(&tar, "tar", "-zcf", "a.tgz", "a.go", "b.go")
+	flag.NewFlagSet(flag.Flag{}).ParseStruct(&tar, os.Args...)
 	fmt.Println(tar.GZ)
 	fmt.Println(tar.Create)
 	fmt.Println(tar.File)
 	fmt.Println(tar.SourceFiles)
 
-	// Output:
+	// Output for `$ go build -o "tar" . && ./tar -zcf a.tgz a.go b.go`:
 	// true
 	// true
 	// a.tgz
 	// [a.go b.go]
 }
-
 ```
 ## Help message
 ```
@@ -146,6 +149,13 @@ Flags:
 
 ## FlagSet
 ```Go
+package main
+
+import (
+	"fmt"
+
+	flag "github.com/cosiner/flag"
+)
 
 type GoCmd struct {
 	Build struct {
@@ -177,8 +187,8 @@ type GoCmd struct {
 	} `usage:"run gofmt on package sources"`
 }
 
-func (*GoCmd) Metadata() map[string]Flag {
-	return map[string]Flag{
+func (*GoCmd) Metadata() map[string]flag.Flag {
+	return map[string]flag.Flag{
 		"": {
 			Usage:   "Go is a tool for managing Go source code.",
 			Arglist: "command [argument]",
@@ -196,10 +206,10 @@ func (*GoCmd) Metadata() map[string]Flag {
 	}
 }
 
-func TestSubset(t *testing.T) {
+func main() {
 	var g GoCmd
 
-	set := NewFlagSet(Flag{})
+	set := flag.NewFlagSet(flag.Flag{})
 	set.StructFlags(&g)
 	set.Help(false)
 	fmt.Println()