Features
* Implement model for error response rendering.
* Implement custom HTTPException, and its handler to have freedom to define returned response body.
* Extend application configuration from environment variables.
* Add and utilize `gunicorn.conf.py` file for better WSGI configuration.
Internal
* Update project dependencies:
* Bump fastapi from 0.66.0 to 0.70.0.
* Bump aioredis from 2.0.0a1 to 2.0.0.
* Bump aiohttp from 3.7.4.post0 to 3.8.1.
* Bump uvicorn from 0.14.0 to 0.15.0.
* Improve submodules import paths.
* Move `fastapi_mvc.app.config` submodule to `fastapi_mvc.config`.
* Refactor application and redis config with using `pydantic.BaseSetting`.
* Extend unit tests, and increase coverage to 99%.
* Change `RedisClient.ping()` method to return false instead of raising an RedisError exception.