Use init.apply() in all class constructors
Converted reader to create array of FeedItem objects

- Much less data to pass around (e.g. to the FeedMenuItem class, since
  the FeedItem object now references the parent reader object). This
  will make things easier when it comes to implementing other features
  which require linkage between feed items and their parent reader (e.g.
  feed aggregation).
- Easier to extend item properties in the future
- Adds some overhead to the load/save_items functions, since they now
  have to convert from FeedItem objects to sanitised objects for storage