Create new base worker CachedElementsWorker

This class would enable a local cache on most high level methods and output Peewee models.

Extends #47 (closed)

unassigned @erwanrouchet

This would greatly simplify the cache handling in base-worker, since most methods that support cache now just have a giant if/else to handle when the cache is enabled and when it isn't, and it would make the static typing and the documentation simpler. This however would be a breaking change for all workers that support cache.

Maybe we should start with… documenting the cache, and figuring out what we really want to do about it.