Add sortable attributes option

[cjilbert504]

Description
In keeping with the common theme of the other MeiliSearch libraries, it would be nice to also have the sortableAttributes option within this library as well.

Basic example

  meilisearch if: :published_at? do
    attribute :category_list, :name, :description, :number
    sortable_attributes [:number]
  end

Other
Any other things you want to add.