Calendar loop: add ability to generate a calendar loop in descending order

As noted by @Birkeholm on the forum: discourse.tangible.one/t/sorting-ordering-months-in-a-calendar-loop/999

In the current version of the plugin, the calendar loop generates years/months/days chronologically (lowest/oldest date to highest/latest date). It’s possible to use the sort_field and sort_order attributes to sort the items in the loop after it’s been generated, but this doesn’t sort the fields within the loop, so even if a year loop is sorted to be in descending order, the months in that loop’s `months` field will still be in ascending order. There needs to be some attribute that works with each calendar loop type that generates everything in reverse chronological order.