Preview mode not supported

In a local/testing environment, there seems to be no way to support the Drip API's [Preview/Test mode](https://developer.drip.com/#preview-test-mode).

I think this would be a great API to have, easily configured with an ENV variable:

```typescript
const drip = Drip({
  token: API_KEY,
  accountId: ACCOUNT_ID,
  preview: true,
})
```