Allow to set button style

**What are you trying to do?**
I would be nice to be able, to apply the SDK button styles: https://developer.paypal.com/sdk/js/reference/#style 

**What's your proposed solution?**
```
paypal_checkout_sdk.Buttons({
  // provided by developer
  style: {
    layout: 'vertical',
    color:  'blue',
    shape:  'rect',
    label:  'paypal'
  }
});
```