Also parse environment variables

[PhoneixS]

Could it be a good addition to be able to also parse environment variables?

For example, you could have a program in which you could specify the host to connect to with a --host 127.0.0.1 or with an environment variable "HOST" with the value "127.0.0.1". This could help when it used in places where they commonly use environment variables like in Helm / Kubernetes.

Nowadays, if you want this behavior you should implement both separately so you can have a problem of not synchronizing it when they change and duplicate validation code, etc.

Could it be a good idea?