Add time_format argument to ExcelWriter.
Currently, converting to Excel a DataFrame containing `datetime.time` instances leads to unformated float values such as `0.666667` instead of `16:00:00`. This pull request fixes this by adding a default time format and adds an argument to specify your own time format.

- needs tests

does this have an issue number?
 @jreback OK for the tests. I guess I only need to modify [these lines](https://github.com/pydata/pandas/blob/7e3212edd642adfd916baf65691dbef5f7b03d0b/pandas/io/tests/test_excel.py#L843-880) to add a time series to `df` & `df_expected`, then add a `time_format='HH-MM-SS'` argument to the writer2 constructor?

There’s no issue related to this pull request. Do I need to create one? That seemed quite obvious to me that `time_format` has only been forgotten.
 ok on the issue number, use this PR number as the issue number in the release note.

Would prefer you add a new set of tests (like those though)
 @BertrandBordage can you update?
 closing as stale, but if you would like to reopen and fixup pls do.
