Expanded string conversion in lib/range.

Slashes are now also accepted as in 06/2012 and output defaults to these if no
day is given. This is because I have not seen anyone writing 6.2012 if June
2012 was meant. The slashy one looks way nicer.
I have tried inserting leading zeros for single-digit months but the look silly
and useless to me.
Added content filter to _list/tabulate.
Simplified filter_documents().
Added experimental content filter.

filter_documents expands nicely and very logical. Right now, I can not remove
keyword restrictions, so it is more a proof of concept or something.
Cleaned up and fixed index page.
Implemented content-filter listing.

This looks great and the best news is that it *almost* works. Content-filters
are stacked in a handy list and may be removed at any time (this is done by
setting cut.remove_me=true and pass this to filter_documents(), this way I
could re-use a vast amount of its logic).
I haven't found out yet why filters are occasionally left in the queue but it
is CLOSING TIME anyways.