Feature Request: Cache download_key_ids

[xcorvis]

When downloading purchased games individually it's necessary to first get the download_key_id. With large libraries this can be many pages, which must be repeated for each individual purchased download. One fix for this would be to download and cache the download_key_ids if it's missing, and re-use the cache when it's available.

See #63 for background.

I have hundreds of purchased games due to a few large bundles, but I don't want everything, and the itch.io official app is so bad at multiple file downloads it's unusable. Ultimately I'd like to be able to create a list of games I want to keep updated and run something like this once in a while:

itch-download -k $key --update-purchases --cache .itchiodl-cache
for $game-url in list-of-games; do
  itch-download -k $key --cache .itchiodl-cache --download-game $game-url
done

[Emersont1]

• If the aim is to keep selected games up to date (provided the required id doesn't change over time), then we may be able to read it from the JSON files written alongside the game.
• Caching the download_key_ids is probably a good solution, but it will stop after it finds it, so it can't guarantee being able to find it if we directly cache the output so would need a separate cache command as suggested