Temporary disable fallbacks

I've made a PR to temporary disable the fallbacks. This is useful when you want to display on an admin page the fields the user hasn't translated, but display on public pages the fallback translations.

Example:

```ruby
p = Post.find(1)
p.name # => 'English'
p.name_nl # => 'English'
p.disable_fallback
p.name # => 'English'
p.name_nl # => nil
```