Options

This adds options to fields and verifies them during .set_value. This logic can be done differently if needed. I branched off of my pr code, so this pr depends on https://github.com/ambitioninc/django-data-schema/pull/45 to be merged first.
Just do

```python
for optional_value in self.fieldoption_set.all()
```

This allows another person to do a prefetch_related on data_schema__fieldschema_set__fieldoption_set, and the self.fieldoption_set.all() query will be cached