Version 3.0.0

* Allow for bulk-loading of feed items. The `ActivityFeed.item_loader`
  has been renamed to `ActivityFeed.items_loader`. The only difference
  is that the proc is now passed an array of IDs, rather than an
  individual ID.
* Updated `item_loader` references to `items_loader` throughout internal
  documentation and README.
* Added `expire_feed_in` method alias for `expire_feed.`