Add option to allow New() to default to creating an XLSX (XML) spreadsheet object

[cfsimplicity]

By default new() creates a binary spreadsheet (.xls) object, but this is the old legacy format which has been superceded by the XML format (.xlsx). XLSX objects support a wider range of functionality and are generally smaller when saved.

In version 5.0 the default format should perhaps change to XML, but for now introduce a configuration option to change the default for new() to XML.