Add Custom Checkout Types

[pololi-stripe]

Summary & motivation

• add Custom Checkout types (so we can use it in https://github.com/stripe/react-stripe-js)
  • I didn't add any documentation to each function/type/interface. Since we are still in beta, this may get changed. We can add the documentation when GA, sounds good?

Testing & documentation

[danwang-stripe]

LGTM!