Adds support for more RSS feeds to xml.feed plugin

This PR updates the xml.feed plugin to support more RSS feed.

Specifically, the PR makes the following improvements:

Checks if the feed ID contains a link that can be used as creator URL.
Uses the first link of an entry as post URL if no link with rel=alternative was found.
Supports publication date residing in entry.updated.
Uses image links as attachments.

The changes were tested with the Apple Newsroom RSS feed. I assume they apply to other RSS feeds too.

TheIconfactory / Tapestry