Custom fields in email notification

Hey, really cool plugin, thanks a lot!

I have one problem though.
I’m trying to add 3 custom fields (which I managed to add in the form and they’re also showing up in the admin).

But I still need to include these custom fields in the email notification.
Of course they’re not showing up with simply adding the placeholder ({review_bonus}) in the email template.

So is there some sort of hook to set those as additional template-tags? I’ve seen the actual tags are set in Notification.php -> buildEmail function.

Thanks in advance & I’d love to hear from you.