Add ability to export in XLS format

Exporting data in XLS format would be very handy:

• It would simplify the Excel import process for users who are not familiar with configuring CSV imports, particularly the selection of the correct encoding (UTF-8) and delimiters.
• Excel does not handle well CSV cells containing multiple lines, as discussed on StackOverflow and more recently on answers.microsoft.com.

In addition, the equivalent feature is available in the Livetable exporter.