Add date/time of report generation in XLS export

[sbidoul]

This could be added at the bottom of the report.

[apineux]

@sbidoul In my opinion, adding the date/time at the end of the report (in a cell) can be bad for end users. If you add that information in a new cell at the end, this can be interpreted as a data of the report instead of a "meta data". What do you think? Can you be more precise in the way you want to display that date/time.

[sbidoul]

The idea is to have a kind of footer saying "Report generated at YYYY-MM-DD HH:MM:SS", perhaps in small italics or something like that.

This is a frequently requested feature to give a more professional look to the output. For PDF exports, this is not an issue because the date/time is usually part of the internal report template.

[sbidoul]