Add support for Embedded Checkout (beta)

[cweiss-stripe]

Summary & motivation

This adds support for Embedded Checkout.

Example usage:

import * as React from 'react';
import {loadStripe} from '@stripe/stripe-js';
import {
  EmbeddedCheckoutProvider,
  EmbeddedCheckout
} from '@stripe/react-stripe-js';

const stripePromise = loadStripe('pk_test_123', {betas: ['embedded_checkout_beta_1']}));

const App = ({clientSecret}) => {
  const options = {clientSecret};

  return (
    <EmbeddedCheckoutProvider
      stripe={stripePromise}
      options={options}
    />
      <EmbeddedCheckout />
    </EmbeddedCheckoutProvider>
  )
}

Testing & documentation

Unit tests and manual testing via storybook.

[cweiss-stripe]

@cyuen-stripe rebased on top of the custom checkout PR #439 -- ready for review!

[cyuen-stripe]

LGTM

[cweiss-stripe]

r? @bmathews-stripe

[bmathews-stripe]

r? @pololi-stripe

^ very similar to your other PR. Can you review?