How to reference specific field with ForeignKeyField
I've got a commit or two towards making this work, almost there.
 Added. http://peewee.readthedocs.org/en/latest/peewee/api.html#ForeignKeyField

I'm trying to find a way to use a `ForeignKeyField` that refers to a specific field of another model, rather than the entire model itself. I believe in Django's ORM it's the [`to_field`](https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ForeignKey.to_field) attribute.

Looking at the [source code](https://github.com/coleifer/peewee/blob/master/peewee.py#L826), there doesn't appear to be any option for this, but rather a check to make sure that `related_name` doesn't conflict with any of the model's field names (`if self.related_name in self.rel_model._meta.fields`...).

If there is no way to currently specify the field, would it be a feasible future enhancement for peewee?
