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.

Reproduce the dict-of-namedtuples behavior in Python Fire, comparing the no-key case with selecting a dict key. Start by tracing the namedtuple handling from the behavior introduced in #161; done means that omitting a key prints all namedtuples rather than a help page listing the dict keys.

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