Provide ways to store cacheable items

This patch contains several new features

- Allow for object storage by using `window.JSON.stringify / parse`
- New extension cache with implement similar D7 cache using a keys entry for a use of a particular cache and keys prepended with their cache id.

Now we can cache ie node/124 for further processing like needed for #60 . this opens up to cache all comments and a processed version into another cache.