Allow the initialization of values

[lyz-code]

Hi, first thanks for your awesome project :)

To build the test cases, I want to create a GoodConf instance with some values that are different from the default, for example the DATABASE_URL attribute. It will be nice that we could do something like:

AppConfig(DATABASE_URL='fake://address')

I know BaseSettings from pydantic supports it, so maybe we can call to super().__init__ in the GoodConf`` init`. What do you think?