Multi field validation (i.e. one of must be present) can result in incorrect `isValid` state

This might be more of a feature request than a bug (at least with the provided validators).

I have implemented a validator which checks for presence OR for the presence of other fields (so at least one of the fields must be "present" for the validator to pass). Currently there is no way to call validation for these other fields as the actual `changeset` is not passed to the validator function (just the changes and the model).

The only "workaround" right now is to call validate on the main changeset validating all fields, but this means that before that happens, the isValid state is incorrect (and errors for said fields might be present on the changeset).

This is likely also an issue for the validator proposed in #189 / #191