Support comparing --file options

[k0kubun]

Example

$ cat before.sql
CREATE TABLE users (
  id bigint PRIMARY KEY NOT NULL,
  created_at datetime NOT NULL
);

$ cat after.sql
CREATE TABLE users (
  id bigint PRIMARY KEY NOT NULL,
  name varchar(40) DEFAULT NULL,
  created_at datetime NOT NULL
);

$ mysqldef --file before.sql --file after.sql
-- dry run --
ALTER TABLE `users` ADD COLUMN `name` varchar(40) DEFAULT null AFTER `id`;