Start migration from specified number

[g105b]

Useful for adding this library to an existing project.

[g105b]

This has already been tested here: https://github.com/PhpGt/Database/blob/7e8bb90f2b2c98a4352ad15575322e0ce1d4139a/test/unit/Migration/MigratorTest.php#L366