Ambient News TODOs:
-------------------

* Make annotations into named constants.
* Read from bookmarks and rank them higher than non-bookmarks.
* Allow user to star and un-star bookmarks from about:news.
* Show tabs, recently closed tabs? Not exactly news-like...
* Prioritize news from open tabs?
* Show less information for lower-priority feeds
* Keep track of what's been clicked on/viewed, politely alert
  reader to new content.
* Consider prioritizing based on 'frecency' (see MDC).
* Don't just take pages into account, also take domains into account.