Add decodeViaJSON, benchmark and example (as a test).

decodeViaJSON performs faster with less memory allocations for simple
uses, despite performing the task using an indirect approach. Use it as
a baseline for comparing performance of mapstructure.Decode.

Credit to @sqs and @gbbr for initial idea.

mapstructure_benchmark_test.go

@@ -1,6 +1,7 @@
 package mapstructure
 
 import (
+	"encoding/json"
 	"testing"
 )
 
@@ -27,6 +28,41 @@ func Benchmark_Decode(b *testing.B) {
 	}
 }
 
+// decodeViaJSON takes the map data and passes it through encoding/json to convert it into the
+// given Go native structure pointed to by v. v must be a pointer to a struct.
+func decodeViaJSON(data interface{}, v interface{}) error {
+	// Perform the task by simply marshalling the input into JSON,
+	// then unmarshalling it into target native Go struct.
+	b, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal(b, v)
+}
+
+func Benchmark_DecodeViaJSON(b *testing.B) {
+	type Person struct {
+		Name   string
+		Age    int
+		Emails []string
+		Extra  map[string]string
+	}
+
+	input := map[string]interface{}{
+		"name":   "Mitchell",
+		"age":    91,
+		"emails": []string{"one", "two", "three"},
+		"extra": map[string]string{
+			"twitter": "mitchellh",
+		},
+	}
+
+	var result Person
+	for i := 0; i < b.N; i++ {
+		decodeViaJSON(input, &result)
+	}
+}
+
 func Benchmark_DecodeBasic(b *testing.B) {
 	input := map[string]interface{}{
 		"vstring": "foo",

mapstructure_examples_test.go

@@ -35,6 +35,37 @@ func ExampleDecode() {
 	// mapstructure.Person{Name:"Mitchell", Age:91, Emails:[]string{"one", "two", "three"}, Extra:map[string]string{"twitter":"mitchellh"}}
 }
 
+func ExampleDecodeViaJSON() {
+	type Person struct {
+		Name   string
+		Age    int
+		Emails []string
+		Extra  map[string]string
+	}
+
+	// This input can come from anywhere, but typically comes from
+	// something like decoding JSON where we're not quite sure of the
+	// struct initially.
+	input := map[string]interface{}{
+		"name":   "Mitchell",
+		"age":    91,
+		"emails": []string{"one", "two", "three"},
+		"extra": map[string]string{
+			"twitter": "mitchellh",
+		},
+	}
+
+	var result Person
+	err := decodeViaJSON(input, &result)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("%#v", result)
+	// Output:
+	// mapstructure.Person{Name:"Mitchell", Age:91, Emails:[]string{"one", "two", "three"}, Extra:map[string]string{"twitter":"mitchellh"}}
+}
+
 func ExampleDecode_errors() {
 	type Person struct {
 		Name   string