Cache tags

[keceli]

This issue is created to follow up on the features @ryanmrichard pointed out during PR #199. Copying from there:

• start a class, say CacheEntry which is just a tag and the pluginplay::detail_::Any instance associated with the tag. See Prune cache #199 (comment).
• some other possible tags are:
  • compress (data can be compressed to save room, but should remain on the heap)
  • archive (it can be written to disk)
    Down the road we could conceivably do some of this automatically by timing how long the module took to run and determining if it's worth saving/compressing the data vs. rerunning the module. See Prune cache #199 (comment).