Support automatic deletion of obsolete cache directories

[kadamwhite]

On a medium-sized project with several bundled assets, the hard sorce cache directory for each configuration hash ends up being around 50mb. This isn't a lot on its own, but after a couple weeks of rapid iteration the overall disk footprint for the parent cache directory ballooned to almost a gig.

This may be out of scope for the plugin, and cleanup steps may certainly be enacted outside of webpack, but I wanted to suggest that it could be helpful to be able to pass a isCacheStale method to the plugin, which could be passed information about the cache file such as how recently a particular cache folder was used or how many have been created since, and return a boolean to be used to remove the cache. That would let us choose whether we want to remove caches after a week, or only keep the latest 3, or some other metric specific to a given project.