Better version of xlsx-to-csv.

Replaced the Perl version with a Python version.  The Python version
uses xlrd library, which seems to be better than the Spreadsheet::XLSX
library.

New version handles Unicode characters correctly.

It has been tested with a large workbook (75M) and can extract a sheet
in about a minute.

The new version has been tested with dates.  A default date format has
been chosen, and a --date-format flag has been added to impose a different
date format.