add ignore field examples
add ignore field examples

faild test result:

```
--- FAIL: TestTableMap__WriteDDL (0.17s)
    tablemap_test.go:46: result is not match:
        DROP TABLE IF EXISTS `product`;

        CREATE TABLE `product` (
            `id` INTEGER unsigned NOT NULL PRIMARY KEY AUTO_INCREMENT,
            `name` VARCHAR(191) NOT NULL,
            `type` INTEGER unsigned NOT NULL,
            `user_id` INTEGER unsigned NOT NULL,
            `received_user_id` INTEGER NULL,
            `description` TEXT NOT NULL,
            `size` INTEGER NULL,
            `created_at` DATETIME NOT NULL,
            `updated_at` DATETIME NULL,
            `-` VARCHAR(191) NOT NULL,
            `` INTEGER NOT NULL,
            UNIQUE (`user_id`, `type`),
            FOREIGN KEY (`user_id`) REFERENCES user(`id`) ON DELETE CASCADE ON UPDATE CASCADE
        ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
        CREATE INDEX product_user_id_created_at ON product (`user_id`, `created_at`);

                        vs
        DROP TABLE IF EXISTS `product`;

        CREATE TABLE `product` (
            `id` INTEGER unsigned NOT NULL PRIMARY KEY AUTO_INCREMENT,
            `name` VARCHAR(191) NOT NULL,
            `type` INTEGER unsigned NOT NULL,
            `user_id` INTEGER unsigned NOT NULL,
            `received_user_id` INTEGER NULL,
            `description` TEXT NOT NULL,
            `size` INTEGER NULL,
            `created_at` DATETIME NOT NULL,
            `updated_at` DATETIME NULL,
            UNIQUE (`user_id`, `type`),
            FOREIGN KEY (`user_id`) REFERENCES user(`id`) ON DELETE CASCADE ON UPDATE CASCADE
        ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
        CREATE INDEX product_user_id_created_at ON product (`user_id`, `created_at`);
```

mackee/go-genddl