Extract all schemas, not just the default one

[pruzko]

It is (typically) possible to extract all schemas from vulnerable web applications, but Hakuin now only extracts the default one, the one that the application is connected to. Supporting extraction of all schemas should only require rewriting the injected queries to take the DB name into consideration. For instance, users will become dbo.users.

[pruzko]

Hakuin can now extract the list of all schemas by invoking Extractor.extract_schema_names() or via hk.py ... -x schemas. Furthermore, all extraction functions now have an extra parameter, to target a specific schema, e.g., Extractor.extract_table_names(schema='dbo') or hk.py ... -s dbo.

Implemented in 64f777f.