LinkedList: Add an append_after() method

Can be used to append an item after another one instead of at the end of the list.
With doubly linked lists, this is extremely easy to achieve (O(1)).

luna/include/luna/LinkedList.h

@@ -47,6 +47,20 @@ template <typename T> class DoublyLinkedList
         m_count++;
     }
 
+    void append_after(T* base, T* ptr)
+    {
+        DoublyLinkedListNode<T>* new_node = (DoublyLinkedListNode<T>*)ptr;
+        DoublyLinkedListNode<T>* base_node = (DoublyLinkedListNode<T>*)base;
+
+        if (m_end_node == base_node) m_end_node = new_node;
+
+        new_node->m_next_node = base_node->m_next_node;
+        base_node->m_next_node = new_node;
+        new_node->m_last_node = base_node;
+
+        m_count++;
+    }
+
     T* remove(T* ptr)
     {
         DoublyLinkedListNode<T>* node = (DoublyLinkedListNode<T>*)ptr;