Add functions to support golang flags in pflags

They aren't "perfect".  spf13/pflag chose to use an explicit `Type` in the
value. golang has nothing of the sort and has a rather crazy
IsBoolFlag() bit of nonsense for the one case that really makes sense to
have the explicit `Type`. Probably because adding interfaces really screws
people up. So supporting golang flags in pflags isn't super super
simple, we need to figure out how to handle our `Type` given only the
information available in a golang flag. This does it as best we can with
`reflect`.

golangflag.go

diff --git a/golangflag.go b/golangflag.go
new file mode 100644
index 0000000..7b36f87
--- /dev/null
+++ b/golangflag.go
@@ -0,0 +1,92 @@
+// Copyright 2009 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 pflag
+
+import (
+	goflag "flag"
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+var _ = fmt.Print
+
+// flagValueWrapper implements pflag.Value around a flag.Value.  The main
+// difference here is the addition of the Type method that returns a string
+// name of the type.  As this is generally unknown, we approximate that with
+// reflection.
+type flagValueWrapper struct {
+	inner    goflag.Value
+	flagType string
+}
+
+// We are just copying the boolFlag interface out of goflag as that is what
+// they use to decide if a flag should get "true" when no arg is given.
+type goBoolFlag interface {
+	goflag.Value
+	IsBoolFlag() bool
+}
+
+func wrapFlagValue(v goflag.Value) Value {
+	// If the flag.Value happens to also be a pflag.Value, just use it directly.
+	if pv, ok := v.(Value); ok {
+		return pv
+	}
+
+	pv := &flagValueWrapper{
+		inner: v,
+	}
+
+	t := reflect.TypeOf(v)
+	if t.Kind() == reflect.Interface || t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+
+	pv.flagType = strings.TrimSuffix(t.Name(), "Value")
+	return pv
+}
+
+func (v *flagValueWrapper) String() string {
+	return v.inner.String()
+}
+
+func (v *flagValueWrapper) Set(s string) error {
+	return v.inner.Set(s)
+}
+
+func (v *flagValueWrapper) Type() string {
+	return v.flagType
+}
+
+func PFlagFromGoFlag(goflag *goflag.Flag) *Flag {
+	// Remember the default value as a string; it won't change.
+	flag := &Flag{
+		Name:     goflag.Name,
+		Usage:    goflag.Usage,
+		Value:    wrapFlagValue(goflag.Value),
+		DefValue: goflag.DefValue,
+	}
+	if fv, ok := goflag.Value.(goBoolFlag); ok && fv.IsBoolFlag() {
+		flag.NoOptDefVal = "true"
+	}
+	return flag
+}
+
+func (f *FlagSet) AddGoFlag(goflag *goflag.Flag) {
+	if f.Lookup(goflag.Name) != nil {
+		return
+	}
+	newflag := PFlagFromGoFlag(goflag)
+	f.AddFlag(newflag)
+}
+
+func (f *FlagSet) AddGoFlagSet(newSet *goflag.FlagSet) {
+	if newSet == nil {
+		return
+	}
+	newSet.VisitAll(func(goflag *goflag.Flag) {
+		f.AddGoFlag(goflag)
+	})
+}

golangflag_test.go

diff --git a/golangflag_test.go b/golangflag_test.go
new file mode 100644
index 0000000..77e2d7d
--- /dev/null
+++ b/golangflag_test.go
@@ -0,0 +1,39 @@
+// Copyright 2009 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 pflag
+
+import (
+	goflag "flag"
+	"testing"
+)
+
+func TestGoflags(t *testing.T) {
+	goflag.String("stringFlag", "stringFlag", "stringFlag")
+	goflag.Bool("boolFlag", false, "boolFlag")
+
+	f := NewFlagSet("test", ContinueOnError)
+
+	f.AddGoFlagSet(goflag.CommandLine)
+	err := f.Parse([]string{"--stringFlag=bob", "--boolFlag"})
+	if err != nil {
+		t.Fatal("expected no error; get", err)
+	}
+
+	getString, err := f.GetString("stringFlag")
+	if err != nil {
+		t.Fatal("expected no error; get", err)
+	}
+	if getString != "bob" {
+		t.Fatalf("expected getString=bob but got getString=%s", getString)
+	}
+
+	getBool, err := f.GetBool("boolFlag")
+	if err != nil {
+		t.Fatal("expected no error; get", err)
+	}
+	if getBool != true {
+		t.Fatalf("expected getBool=true but got getBool=%v", getBool)
+	}
+}