Yield model instance in __static_column__ keyword's block

This gives `__static_column__` keyword more flexibility. For example, example in #41 can be written like:
```ruby
comma do
  name
  description

  publisher :name

  publisher.custom_data.each do |header|
    __custom_column__ header do |o| o.custom_values[header] end
  end
end
```
I think that this is cleaner.