Pass instance to I18n when translating error

It'll allow to do like this:
```ruby
# inside model that has .title
validates_numericality_of :quantity, greater_then: 0,
  title: ->(h) { h[:instance].title }
```
with translations
```yaml
errors:
  models:
    my_model:
      greater_then: of %{title} must be greater than %{count}
```