blog-posts macro: add parameter for date format

If you use the blog-posts macro with the content=titles parameter, the macro displays 3 columns: title, author and date. The date column displays date and time. As far as I'm concerned, just the date is enough. That would make the macro's output look a lot better if it's displayed inside a column of 50% width (date/time is now being wrapped). So it would be a good improvement if the date format can be specified as a parameter to this macro.