Improve heuristics for selecting a link URL.

Many atom feeds have multiple `link` elements, some of which point to resources other than web pages.  This patch tries to find the best feed based on the `type` attribute.

Example of feed that is currently broken: http://googlenewsblog.blogspot.com/feeds/posts/default

The atom package cannot access the type attribute of link elements, so I added a tweaked version at `goapp/atom`.