Add support for sorting the feed

[AshleyButcher]

Not all feed generators are kind enough to order items in a logical order, and not all feeds are in the order required for consumption on a site.

To this end, it would be useful to support the sort attribute on the feed tag in the same manner as with standard Textpattern list tags (such as category_list).

Ideally this would be possible as a part of the XPath query, however XPath 3.1 functions like fn:sort are not available via PHP's SimpleXML since it's just using libxml2 (= XPath 1.0).

Problems to overcome:

• The way items are output will need to be refactored to add an intermediary stage when sorting is active which would need to convert the XML into an array for a sort/multisort operation prior to output.
• Sorting on custom fields will likely need to have the limitation of only being on tag level (not attributes)
• Sorting on common fields such as the publication date (probably the most common use case) will need to have a common alias as different feed formats have different naming (RSS vs. Atom as the obvious example).