Add support for sequential introduction of a category

This allows a particular category to be configured so that new items
will be introduced in the order they exist in the file, rather than
randomly, (though once an item leaves bin 0 it gets the same random
treatment as all other items).

The behavior is controlled by a new option at the beginning of the
file that looks like this:

	order = sequential

If there is no option it will be treated as random, (and when written
out the order option will be set to either sequential or random).