Implement migrations for adding missing columns and other schema modifications
When adding new fields to objects, reusing the same database throws the "column missing" error. For sqlite new columns can be added with the [ALTER TABLE](https://www.w3schools.com/sql/sql_alter.asp) command. It would be great if this could either be done automatically, or explicitly via a function call, not sure what the best approach would be. This would prevent the need for migration scripts which can grow out of hand for cases where lots of fields get added over time. Since norm aims to make the usage of databases from Nim more convenient, I think this would be a welcome addition.