New: /oldest/ virtual directory restriction

DWiki has long been able to give people the latest N things in a virtual directory context (as latest/<N>). Now it can give them the oldest N things, using the obvious syntax: oldest/<howmany>.

Just to show off, ranges properly convert themselves into 'oldest/<N>' at the end of their run, just as they convert themselves into 'latest/<N>' at the start.

Documentation has been updated appropriately.