Allow more flexibility in RC feeds

The user can now specify which feeds to send to in
RecentChange::notifyFeeds if they don't want to use
$wgRCFeeds.

Additionally, the 'formatter' in $wgRCFeeds can now be an object
rather than just a class name.

Change-Id: Ibfdffc17a934e35223887c123331795563102752
Merge "Allow more flexibility in RC feeds"