Changes
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* Added plugin system, see updated [documentation](https://github.com/Fatal1ty/openapify#plugins)
* Fixed bug when `content` section was not added when `media_type` attribute was used without `body_type` in `request_schema` and `response_schema` decorators
* Improved the default logic for determining the body media type, now it chooses `application/octet-stream` for `bytes` and `bytearray` instead of `application/json`