Adding support for %{count}

[milgner]

There are some use cases where I'd like to use the count functionality so that if you have an enum model_type and then you want to show a list of records of model type "Example", you can translate it to "My Examples" - but if there's only one in the list, it would just say "My Example".

If there aren't any arguments against it, I'd prepare a PR to enhance the translated_[enum_attribute] and corresponding methods with an additional argument to specify count (and potentially other arguments, I'd just pass them on to I18n.t).