Possibility of specifying additional html attributes to 'wrap'

@remofritzsche created a [pull request](https://github.com/plataformatec/simple_form/pull/899) to add an additional option named ```html``` that contains additional html attributes to be used on wrapper. 

Example:

```
config.wrappers do |b|
  b.wrapper :tag => :td, :class => :some_class, :html => { :additional_html_attribute => :value }
end
```

I've just added some tests to guarantee the feature works.

Thanks for the PR @remofritzsche.
Thanks for the help @bellthoven @rafaelfranca.