Allow custom time format parsing

I would like to retrieve information of the database on a time variable using a specific format. The [Go-MySQL-Driver](https://github.com/go-sql-driver/mysql) uses the format ["2006-01-02 15:04:05"](https://github.com/go-sql-driver/mysql/blob/master/const.go#L14), and the go-testdb by default uses RFC3339.