Cache tags

This feature supersedes the existing Cache sections feature. It replaces sections with tags, and allows multiple tags to be added to a Cache, with the cache removed if any one of the tags is flushed.

See the intended use here:
https://gist.github.com/karptonite/7093489

Multiple tags are not implemented for the Redis engine (tag/section still works), and are not implemented at all for engines which did not support sections (file and database).