Rather than just using a string for PubDate, we attempt to parse it.
This includes a couple of crazy non-standard time formats that I've seen
in the wild.
Breaking change: Item.PubDate is no longer a string, it is time.Time.
Due to recent changes involving how values are dealt with in xmlx, update
the RSS/Atom parsing. Instead of using the Value property of an xmlx Node,
use the new GetValue function offered in PR jteeuwen/go-pkg-xmlx#15.
The haveItem check for RSS and Atom causes the feeds to act unexpectedly.
For RSS, the checked tags don't necessarily have to be unique. For atom,
it is allowed to have duplicate items (including duplicated ids) in one feed,
so this shouldn't be stopped either.