Add row_attributes

Add ability to specify options/attributes on rows via args or block, eg:

```ruby
class SomeTable < DefaultTable

  row_attributes do |obj|
    {class: "danger"} if obj.is_dangerous?
  end

  column :whatever

end
```
or
```ruby
class SomeTable < DefaultTable

  row_attributes class: "danger"

  column :whatever

end
```