Allow properties to be populated from environment variables

[jpdillingham]

Configuration might look like this:

        [Argument('p', "password", enVar: "MYAPP_PASSWORD")]
        private static string Password { get; set; }

Alternatively a breaking change can be introduced to insert the enVar argument before helpText, which would be more natural.

Environment variable values would be populated only if the argument is not present in the command line arguments.

[jpdillingham]

This is pretty orthogonal to the rest of the lib. Maybe I'll make a separate project for it later.