v0.3.0

• Refactoring migrate logic, and this version is not compatible with previous version.
• Now there don't need old_models.py and it store in database.
• Upgrade steps:
  1. Upgrade aerich version.
  2. Drop aerich table in database.
  3. Delete migrations/{app} folder and rerun aerich init-db.
  4. Update model and aerich migrate normally.