Additional parameters

Needed to append some additional parameters to the generated sortable urls, e.g.
```ruby
<th><%= sortable_column "Name", :column => "name", :additional_params => {:start_date => "#{@start_date}", :end_date => "#{@end_date}", :remote => true} %></th>
<th><%= sortable_column "Total spend", :column => "total_spend", :additional_params => {:start_date => "#{@start_date}", :end_date => "#{@end_date}", :remote => true} %></th>
<th><%= sortable_column "CPC", :column => "cpc", :additional_params => {:start_date => "#{@start_date}", :end_date => "#{@end_date}", :remote => true} %></th>
```