Add support for helper methods

[SpencerKaiser]

Pre-requisites:

• [x] Yes, I looked through both open and closed issues looking for what I needed

Feature Description

We should add support for helper methods. Here's @johnkahn's suggestion:

import setEnv, { key } from '../..';
const env = setEnv({
  nodeEnv: key('NODE_ENV').required().asString(),
  optional: key('PORT').default(3000).asPortNumber(),
});