allow to show events in archive

This adds an option `showEventsInArchive` very similar to `showEventsInMainLoop`.

Please note that this is not fully implemented. I wanted to ask if you are interested before wasting time on this.

Another note: If https://core.trac.wordpress.org/ticket/15202 was committed the `getarchive_where` filter could also be added to `getcalendar_where`.