Default value from ENV

[mhf-ir]

Like: https://cli.urfave.org/v2/examples/flags/#values-from-the-environment

For instance:

type userOpts struct {
	Int  int    `flag:"name=int0;shorts=i;required=true;env=INT_VALUE_KEY;desc=int option message"`
	Bol  bool   `flag:"name=bol;shorts=b;env=BOOL_VALUE_KEY;desc=bool option message"`
	Str1 string `flag:"name=str1;shorts=o,h;env=STRING_VALUE_KEY;required=true;desc=str1 message"`
}