Support `env_prefix`

[lyz-code]

Hi, I'm trying to use the BaseSettings env_prefix Config property to preprend a string on the environment variables without success. A simple snippet that shows the desired behaviour is:

import os

from goodconf import GoodConf


class AppConfig(GoodConf):
    "Configuration for My App"
    DEBUG: bool = False

    class Config:
        env_previx = "GOOD_"


os.environ["GOOD_DEBUG"] = "True"

config = AppConfig(load=True)

assert config.DEBUG

Using os.environ['DEBUG'] = "True" works though