Add a warn option

[TheLox95]

Hi all I think it would be helpful to have the option to define a custom warning message that would be show on console when a var is not defined

import { cleanEnv, str } from "envalid";

const env = cleanEnv(process.env, {
  EMAIL_SMTP: str({
    warning: "EMAIL_SMTP is not set. App will run but it will not send any email.",
  }),
});