[Feature Request] Add Alter Table

[ka-ramba]

When migrating data from Sqlite to Postgres for example the available options allow creating tables in the destination db or using the existing ones. The latter option assumes that the destination db has all tables and columns needed for migration. A big set of use cases however is to migrate data into the existing db that is not fully compatible with the source. For such cases pgloader could call alter table in case the destination table does not contain the required columns. As of notw the caller needs to do this work itself and detect the source and destination db structure, essentially reimplementing the functionality of pgloader.