Checks migration name in table

[Raiden1974]

Better checks migration name in table rather than the count applied migration.

[lopezator]

Hello @Raiden1974 !! Could you give more context about your use case? To better understand your request and see what we can do.

[Raiden1974]

Here:

	// count applied migrations
	count, err := countApplied(db, m.tableName)
	if err != nil {
		return err
	}

	if count > len(m.migrations) {
		return errors.New("migrator: applied migration number on db cannot be greater than the defined migration list")
	}

You checks count of applied migrations. I think what is better checks table for migration name exists. And exec only not exists migrations.