Add support for a per-request options hash in the place of an API key.
For example, when creating a customer, you can specify an idempotency
key:

```
stripe.customers.create({
  description: 'Some customer'
}, {
  api_key: 'sk_test_xxxxx',
  idempotency_key: 'foo'
});
```
This allows us to add more per-request attributes in the future.