Allow tweaking of a cloned object prior to showing its admin form

This allows for twaks prior to diplaying the cloned object in its admin
form. For example, you could append " (duplicate)" to the object name.

NOTE: I added a sample use of the new feature in the sample project to advertise its use, but that had rather important consequences in the tests. Do you think it's too much? We could remove the sampleproject example, but then I don't know where we would document it. I'm a bit afraid to overcrowd the README (and we don't have full docs yet).
Your sample code works fine, but only for the first inline. I never realized that until now but you could have more than one inline with the same model

```python
class PostAdmin(ClonableModelAdmin):
    inlines = CommentInline, CommentInline
``` 

of course that is not useful unless I use different `get_queryset` methods for each inline. So in this case, `tweak_cloned_inline_fields()` will be called twice, first time with `comment_set` and second time with `comment_set-2`