Add support for caching game details
Helps with large libraries when running the downloader multiple times.
Getting game details for many games takes a long time. Caching the game details makes the process much faster for subsequent runs.

Game details are cached to "$XDG_CACHE_HOME/lgogdownloader/gamedetails.json"
--update-cache creates and updates the cache.
--use-cache enables loading game details from cache.
--cache-valid specifies how long cached game details are considered valid