added a tiny utility: show_item_id.rb

The utility shows Item ID from a header file contains a enum block.
Current implementation is mostly for ItemEnum.h.

<< Example to use>>
$ ./show_item_id.rb ../../src/ItemEnum.h
----: /* Asura
----:    Copyright (C) 2013 MIRACLE LINUX CORPORATION
----:
----:    This program is free software: you can redistribute it and/or modify
<snip>
----: enum
----: {
0000:   ITEM_ID_NOT_SET,
0001:   ITEM_ID_NOBODY,