Ability to export reports as spreadsheets

[parafoxia]

Summary
analytix can currently export reports to JSON and CSV/TSV formats, though being able to export as a spreadsheet (XSLX by default) would be useful in cases where data will be analysed in Excel.

The ideal implementation
A Report.as_spreadsheet or Report.as_excel method. Potentially allow for different spreadsheet extensions to be allowed aside from XSLX.

Contribution
Do you want to contribute this feature yourself?
Yes/No