add a force cache refresh switch

[bbelderbos]

We now manage it with CACHE_EXPIRATION_SECONDS env variable (currently on cache branch), but it would be nice to have a --nocache switch (or similar) if there is content on Pybites that has not been pulled in yet by the cache. Only limitation is that the endpoints on codechalleng.es also have a daily cronjob / scraping delay so you might not always get the 100% latest, but giving the user this option would be useful.