Add delete command

The delete command removes an item from the todo-list, it does this by
filtering out the item wich has the given `id' value and saving the
result.

undone/main.scm

@@ -157,6 +157,14 @@
                            (cons var val)))
                        args))))))
 
+(define (delete args)
+  "Delete the specified element."
+  (set! todo-list
+        (filter (lambda (elm)
+                  (not (eq? (get-field-value id elm)
+                            (string->number (car args)))))
+                todo-list)))
+
 (define-view default
   "Default todo-list view, show only the title."
   "~2d ~a~%" id content)
@@ -188,6 +196,9 @@
         ((add)
          (add (list-tail args 2))
          (save))
+        ((delete)
+         (delete (list-tail args 2))
+         (save))
         ((view)
          (view (list-tail args 2)))
         ((views)