Add support for environment variables

[ake-persson]

Would be nice to have support for environment variables built-in like:

--my-opt=<value>                     My option (env: MY_OPT). [default: my-value]

[ake-persson]

Basically the equiv. of doing:

if v, ok := args["--my-opt"]; ok && v == nil {
	if v, ok := os.LookupEnv("MY_OPT"); ok {
		args["--my-opt"] = v
	}
}