Add support for reading values from the environment variables.

[lechgu]

In my projects I often configure the default option values through the environment variables. specifying the option explicitly overrides the environment variable.

Something like this would be great: [Option(Description = "Port to listen on", EnvVar="PORT")]

It is not that hard to capture these manually, but it is a boilerplate code I have to write again and again.

Optionally: support .env files with something like https://www.nuget.org/packages/dotenv.net/