Add option on how to handle orphans when arranging nodes

lib/ancestry/class_methods.rb

@@ -40,14 +40,14 @@ def arrange options = {}
     # @param nodes [Array[Node]] nodes to be arranged
     # @returns Hash{Node => {Node => {}, Node => {}}}
     # If a node's parent is not included, the node will be included as if it is a top level node
-    def arrange_nodes(nodes)
+    def arrange_nodes(nodes, add_orphans: true)
       node_ids = Set.new(nodes.map(&:id))
       index = Hash.new { |h, k| h[k] = {} }
 
       nodes.each_with_object({}) do |node, arranged|
         children = index[node.id]
         index[node.parent_id][node] = children
-        arranged[node] = children unless node_ids.include?(node.parent_id)
+        arranged[node] = children if node.parent_id.nil? || (add_orphans && !node_ids.include?(node.parent_id))
       end
     end