Support go migrations

[denismakogon]

SQL migrations is a good thing, but likely useless in many cases. Migrate must support go migrations, i.e a function that accepts an instance of the database or a transaction object (in case of SQL datastores).
Something along the lines of this to up the version:

func up(tx *sql.Tx) error {
         _, err := tx.Exec("ALTER TABLE calls ADD stats text;")
	return err
}

down also looks the same:

func down(tx *sql.Tx) error {
	_, err := tx.Exec("ALTER TABLE calls DROP COLUMN stats;")
	return err
}

original issue: mattes/migrate#344

[dhui]

Duplicate of #15
Use that issue for discussion