Update: WhatsApp has implemented heads-up notification natively. However, users can still use the apps listed here to display heads-up notification for other apps. If the user is already using it, don't forget to disable WhatsApp's entry to prevent double heads-up notification.
At that time, it's impossible to do this directly from WhatsApp official app.
The heads-up notification is only shown when the notification is set to high-priority, max-priority, or full-screen notification by the developer. So, the "official" answer was to wait for them integrate the function.
However, there are 3rd-party apps that implement this feature for all apps:
- Heads-up notifications
- Heads Up! - notifications
You can control which apps are permitted to show the heads-up notification (I recommend you to do that before they flood the screen), and the customization is also pretty extensive.