Add db.Transaction method for create Transaction block. (go-gorm#2767)

* Add `db.Transaction` method for create Transaction block.

example:

```go
func CreateAnimals(db *gorm.DB) error {
  db.Transaction(func(tx *gorm.DB) error {
    if err := tx.Create(&Animal{Name: "Giraffe"}).Error; err != nil {
      // return any error will rollback
      return err
    }

    if err := tx.Create(&Animal{Name: "Lion"}).Error; err != nil {
      return err
    }

    // return nil will commit
    return nil
  })
}
```

* Ensure rollback when commit has error.

Author: huacnlee

main.go

@@ -525,6 +525,31 @@ func (s *DB) Debug() *DB {
 	return s.clone().LogMode(true)
 }
 
+// Transaction start a transaction as a block,
+// return error will rollback, otherwise to commit.
+func (s *DB) Transaction(fc func(tx *DB) error) (err error) {
+	tx := s.Begin()
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("%s", r)
+			tx.Rollback()
+			return
+		}
+	}()
+
+	err = fc(tx)
+
+	if err == nil {
+		err = tx.Commit().Error
+	}
+
+	// Makesure rollback when Block error or Commit error
+	if err != nil {
+		tx.Rollback()
+	}
+	return
+}
+
 // Begin begins a transaction
 func (s *DB) Begin() *DB {
 	return s.BeginTx(context.Background(), &sql.TxOptions{})

main_test.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"database/sql"
 	"database/sql/driver"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -469,6 +470,65 @@ func TestTransaction(t *testing.T) {
 	}
 }
 
+func TestTransactionWithBlock(t *testing.T) {
+	// rollback
+	err := DB.Transaction(func(tx *gorm.DB) error {
+		u := User{Name: "transcation"}
+		if err := tx.Save(&u).Error; err != nil {
+			t.Errorf("No error should raise")
+		}
+
+		if err := tx.First(&User{}, "name = ?", "transcation").Error; err != nil {
+			t.Errorf("Should find saved record")
+		}
+
+		return errors.New("the error message")
+	})
+
+	if err.Error() != "the error message" {
+		t.Errorf("Transaction return error will equal the block returns error")
+	}
+
+	if err := DB.First(&User{}, "name = ?", "transcation").Error; err == nil {
+		t.Errorf("Should not find record after rollback")
+	}
+
+	// commit
+	DB.Transaction(func(tx *gorm.DB) error {
+		u2 := User{Name: "transcation-2"}
+		if err := tx.Save(&u2).Error; err != nil {
+			t.Errorf("No error should raise")
+		}
+
+		if err := tx.First(&User{}, "name = ?", "transcation-2").Error; err != nil {
+			t.Errorf("Should find saved record")
+		}
+		return nil
+	})
+
+	if err := DB.First(&User{}, "name = ?", "transcation-2").Error; err != nil {
+		t.Errorf("Should be able to find committed record")
+	}
+
+	// panic will rollback
+	DB.Transaction(func(tx *gorm.DB) error {
+		u3 := User{Name: "transcation-3"}
+		if err := tx.Save(&u3).Error; err != nil {
+			t.Errorf("No error should raise")
+		}
+
+		if err := tx.First(&User{}, "name = ?", "transcation-3").Error; err != nil {
+			t.Errorf("Should find saved record")
+		}
+
+		panic("force panic")
+	})
+
+	if err := DB.First(&User{}, "name = ?", "transcation").Error; err == nil {
+		t.Errorf("Should not find record after panic rollback")
+	}
+}
+
 func TestTransaction_NoErrorOnRollbackAfterCommit(t *testing.T) {
 	tx := DB.Begin()
 	u := User{Name: "transcation"}