Introducing application 'atexit' hook.

Finalizing Python interpreter.

This closes #65 issue on GitHub.

src/nxt_application.h

@@ -218,6 +218,7 @@ struct nxt_app_module_s {
     nxt_int_t                  (*run)(nxt_task_t *task,
                                     nxt_app_rmsg_t *rmsg,
                                     nxt_app_wmsg_t *wmsg);
+    void                       (*atexit)(nxt_task_t *task);
 };