v0.7.0

Now aerich use .py file to record versions.

Upgrade Note:

1. Truncate aerich table
2. Delete migrations/models folder
3. Run aerich init-db

• Improve inspectdb adding support to postgresql::numeric data type
• Add support for dynamically load DDL classes easing to add support to
  new databases without changing Migrate class logic
• Fix decimal field change. (#246)
• Support add/remove field with index.