title: Implement custom function on Django search_fields
body: <p>I have a model with an ISBN field.
  ISBN (International Standard Book Number) may be ISBN10 (10 digits) or ISBN13 (13 digits). I can convert 10 to 13 and 13 to 10 with a custom function.
  The point is I want Django to search in ISBN field, no matter if it is ISBN10 or ISBN13 (the field accepts both). To do that, it is necessary to include the convert function somewhere. For example: if the value of my search is 8489691207 I want Django to search that value and the converted value to ISBN13 (9788489691209).</p>
  
  <p>I'm using Django 1.8 and Python 2.7</p>