Ability to specify a cached tag as global

Right now, all tag caches are tied to the URI, which creates an unnecessary number of cache files for some content such as global headers, footers, and sidebars. For example, a list of recent or popular posts, or navigation which queries categories, etc - these items can be the same for every page site-wide. On large sites with thousands of articles, this results in a huge number of unnecessary queries and cache files.

Something like a new parameter called globalwhich, when set to yes, would omit the URI from the pre-hash cache name when both setting and getting the cache.