title: search_field by field choice
body: <p>I've a basic django model and i was wondering is there a way to make the search fields search in the value i am mapping to instead of searching in the value saved in the database, is there a possible way that i can search by the value "Premium" ? </p>
  
  <p>Model.py</p>
  
  <pre><code>class User(models.Model):
      account = models.ForeignKey('Account')
      name =models.CharField(max_length=50)
      ACCOUNT_CHOICES = (
         (1, 'Premium'),
         (0, 'Normal'),)
      type = models.CharField(choices=ACCOUNT_CHOICES)
  </code></pre>
  
  <p>Admin.py</p>
  
  <pre><code>class UserAdmin(admin.ModelAdmin):
       search_fields = ['name','type']
      pass
  admin.site.register(User,UserAdmin)
  </code></pre>