Add a SetEnvVars API method

[stephenh]

Hi! Currently I'm using env-cmd to load .env for script commands in package.json, but when tests are invoked from vscode/webstorm, I'm using Jest's globalSetup with this file:

import { GetEnvVars } from "env-cmd";

export default async function globalSetup() {
  Object.entries(await GetEnvVars()).forEach(([key, value]) => (process.env[key] = value));
}

I've copy/pasted this incantation into a few projects now, and I'd like to be able to do just:

import { SetEnvVars } from "env-cmd";

export default SetEnvVars;

Does that seem reasonable? Happy to work on a PR if so. Thanks!