Add ErrorUnused config where it is an error if there are unused keys

mapstructure.go

diff --git a/mapstructure.go b/mapstructure.go
index 151741c..231df40 100644
--- a/mapstructure.go
+++ b/mapstructure.go
@@ -11,12 +11,18 @@
 	"errors"
 	"fmt"
 	"reflect"
+	"sort"
 	"strings"
 )
 
 // DecoderConfig is the configuration that is used to create a new decoder
 // and allows customization of various aspects of decoding.
 type DecoderConfig struct {
+	// If ErrorUnused is true, then it is an error for there to exist
+	// keys in the original map that were unused in the decoding process
+	// (extra keys).
+	ErrorUnused bool
+
 	// Metadata is the struct that will contain extra metadata about
 	// the decoding. If this is nil, then no metadata will be tracked.
 	Metadata *Metadata
@@ -372,6 +378,17 @@
 		}
 	}
 
+	if d.config.ErrorUnused && len(dataValKeysUnused) > 0 {
+		keys := make([]string, 0, len(dataValKeysUnused))
+		for rawKey, _ := range dataValKeysUnused {
+			keys = append(keys, rawKey.(string))
+		}
+		sort.Strings(keys)
+
+		err := fmt.Errorf("'%s' has invalid keys: %s", name, strings.Join(keys, ", "))
+		errors = appendErrors(errors, err)
+	}
+
 	if len(errors) > 0 {
 		return &Error{errors}
 	}

mapstructure_test.go

diff --git a/mapstructure_test.go b/mapstructure_test.go
index cc1f249..b3e1363 100644
--- a/mapstructure_test.go
+++ b/mapstructure_test.go
@@ -123,6 +123,31 @@
 	}
 }
 
+func TestDecoder_ErrorUnused(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"vstring": "hello",
+		"foo": "bar",
+	}
+
+	var result Basic
+	config := &DecoderConfig{
+		ErrorUnused: true,
+		Result:   &result,
+	}
+
+	decoder, err := NewDecoder(config)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	err = decoder.Decode(input)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}
+
 func TestMap(t *testing.T) {
 	t.Parallel()