Add options :scope for nestable_list

Hi! Because the `nestable` list is ascending sort by `position_field`, but normally it is null or the default value on database (excepts sorted via `nestable`), and `rails_admin` list is descending sort by primary key. So I have feature plan implementing scope options to help sorting them or whatever.

Example:

```ruby
# models/list.rb
class List < ActiveRecord::Base
  # ...
  scope :by_default, -> { List.order('id desc') }
  # ...
end

# config/initializers/rails_admin.rb
RailsAdmin.config do |config|
  # ...
  config.model 'List' do
    nestable_list({
      scope: :by_default # or scope: -> { List.order('id desc') }
    })
  end
  # ...
end
```
and on the `nestable` list page have like this SQLs:
```sql
SELECT * FROM `lists` ORDER BY position, id desc
```