Filterable RSS/Atom feed endpoint

It would be nice to have a feed endpoint that can filter the apps listed in the feed. Many admins restrict the apps available to their NC instances, so being alerted for updates to just those apps would be useful.

This could be done by passing the app names as a URL parameter like `?apps=mail+contacts+calendar+passwords` (whitelist) or `?apps=-mail-contacts-calendar-passwords` (blacklist).

There are a few third party websites which provide this functionality, but having it built-in would be handy.