title: Prewarming several images in same model with django-versatileimage
body: <p>I'm successfully using VersatileImage for my django project to prewarm images (i.e. save images at various sizes postsave), but I've come up against a situation where I want my model to have several image fields and I'm not sure how to prewarm them as the code I'm using only seems to allow a reference to a single image field.</p>
  
  <p>models.py</p>
  
  <pre><code>class Testimonial(models.Model):
      author = models.ForeignKey(User, on_delete=models.CASCADE, default=None)
      image = VersatileImageField('image_testimonial', upload_to=upload_location, validators=[file_size], null=True, blank=True)
      image2 = VersatileImageField('image_testimonial', upload_to=upload_location, validators=[file_size], null=True, blank=True)
      message = models.TextField(default='', null=True, blank=True)
  
      def __str__(self):
          return self.author.username    
  
  @receiver(models.signals.post_save, sender=Testimonial)
  def warm_testimonial_images(sender, instance, **kwargs):
      testimonial_img_warmer = VersatileImageFieldWarmer(
          instance_or_queryset=instance,
          rendition_key_set='image_testimonial',
          image_attr='image'
      )
      num_created, failed_to_create = testimonial_img_warmer.warm()
  
  @receiver(models.signals.post_delete, sender=Testimonial)
  def delete_testimonial_images(sender, instance, **kwargs):
      instance.image.delete_all_created_images()
      instance.image.delete(save=False)
  </code></pre>
  
  <p>This code will only prewarm images that are uploaded to the 'image' file on the model on account of this line:</p>
  
  <pre><code>image_attr='image'
  </code></pre>
  
  <p>I'm not sure how to adapt this code to handle image2 as well.</p>
  
  <p>Thanks</p>