Backreference relationship signal
I do not think that I would merge such functionality into peewee, but if you end up implementing it on you r own I'd love to see the diff. Maybe you could post it to this issue?
 Haven't implemented yet, but I'm imagining this. This is not using the signals architecture as I'm not too well-read on your implementation of it.

```
# Save method on the base model (from which everyone inherits)
def save(self, *args, **kwargs):
    # Check all our ForeignKeyFields...
    for field in self.dirty_fields():
        if isinstance(field, ForeignKeyField):
            # If we're using signals, I think we would look it up and fire it here instead of making a
            # heavy assumption that the rel_model implements the method fk_established(self)
            try:
                field.rel_model.fk_established()
            except AttributeError:
                pass
```

Hey! I have a suggestion for an addition (and perhaps I could even undertake implementation if you think it's worthwhile and would accept changes!) to the signal support in `playhouse`. It's a little hard to articulate, but I think there might be value in a signal that fires for some model instance, let's call it `Banana`, when any other model establishes a Foreign Key relationship to that instance. I can imagine that in the save handler of a Model, we'd look at the fields for any dirty FKs, and fire a signal at them.

Does that make sense? Do you think that would be useful?

(the other option, of course, is for me to just implement that logic local to _my_ base model class and not make it part of the signal architecture)
