make model_options inheritable

This PR makes model_options inheritable, so that subclassing a Form with a model setting will be preserved in the subclass.

```ruby
class UserForm < Reform::Form
  model :some_user
end

class AdminUserForm < UserForm
end

puts UserForm.model_name.to_s #=>  "SomeUser"
puts AdminUserForm.model_name.to_s #=> "SomeUser"
```