feat: support skip table by setting managed=False

[waketzheng]

Close #299

TODO

• Add unittest
• Update old_models_describe
• Handle case that change managed=False to managed=True after several migrations
• Update changelog after v0.8.1 released
• Merge commits into one
• Request review

Description

1. if getattr(MyModel.Meta, 'managed', True) is False, ignore MyModel in migration file
2. model describe dict will save to the aerich table with a extra "skip": True key value
3. when change managed from false to true, generate migration base on the old_models_describe that skip is not True