Allow involved processor to change the cache key

While many processor can afford using cached assets as long as the sources didn't change some needs to invalidate all the caches between versions. 

E.g.: currently when [Opal](https://github.com/opal/opal-rails#gotchas) is upgraded and a file store is used the cache store folder must be removed (i.e. tmp/cache/assets under rails).

If it's conceptually ok I'll add tests

cc @adambeynon