Multiple schema support

[r614]

Problem

--schema only accepts one schema. Users who need to diff multiple schemas must run migra multiple times. Upstream issue djrobstep#190.

Fix

• Support schemas: list[str] | None at the Migration class level
• Modify get_inspector to accept a list of schemas
• CLI: --schema with action="append" so --schema s1 --schema s2 works
• Backward compatible

Files: schemainspect get/obj/SQL, migra.py, command.py