Breaking change. New parsing .env files logic

[wawan93]

What changed

1. ReadConfig only parses files, and doesn't load env variables.
2. ReadConfig with .env file doesn't change os envs.
3. ReadEnv updates fields, read earlier from ReadConfig

Why

As stated in Contributing.md,

... this project is designed to be minimalistic, obvious, and as simple as possible. We prioritize clear and easy-to-understand code over complex or "magical" solutions.

But, right now the behavior of ReadConfig is not obvious at all. And, in case with .env - it's "magical". When I see two methods ReadEnv and ReadConfig, I expect that ReadEnv reads config only from envs, not from files; and ReadConfig only reads from a file, not from env variables. And If I need to do both, I should call both methods. But if I call ReadConfig with .env file, I can't read env variables anymore, because they are changed.

I suggest changing this behavior: make ReadConfig read only from the file, not from the env; and make it work correctly with .env files (not to change os.GetEnv behavior).