Possibility of specifying additional html attributes to 'wrap'

Currently, when specifying a wrapper using the `wrap` method, the only HTML attribute supported is `class`. This simple change allows to specify an additional option named `html` which can contain additional HTML attributes like such:

```ruby
config.wrappers do |b|
  b.wrapper :tag => :td, :class => :some_class, :html => { :additional_html_attribute => :value }
end
```

I'd love to see this feature in SimpleForm :)