{WIP} Implement schema editing.

[dimitri]

Add a facility to dump DDL commands for a specific set of tables, and include also their indexes and the foreign key definitions that point to selected tables. The command is pgcopydb dump sql-files --filters ...

Each SQL object definition is saved in their own file: .sql

Later implement a way to use the saved files instead of the pg_dump archive contents when preparing the schema on the target database, in a way that if the SQL files have been edited the new schema is installed there.

See #263.