Display dict of namedtuples

Fire nicely prints a dict of dicts using one line per inner dict. When I try to do the same with dict of namedtuples (which otherwise behave like dicts after #161), I just get a help page listing all the dict keys. When I specify a dict key, then that namedtuple is printed. I would prefer all namedtuples in the dict to be printed when no key is specified.

重現 Python Fire 中 dict-of-namedtuples 的行為，比較不指定鍵的情況與選擇 dict 鍵的情況。首先從 #161 引入的行為開始追蹤 namedtuple 的處理；當省略鍵時輸出所有 namedtuples，而不是顯示列出 dict 鍵的說明頁面時，即表示完成。

由索引模型根據 Issue 內容生成。