Add helper script for URLCaches

[nichtich]

• Update all (cache invalidation, see Cache invalidation #8 )
• Remove from cache (purge)
• add entry to cache (for Support retrieving schema files from URL #9)
• List retrieved URLs:

jq -r '[.hash,.url]|@tsv' urlcache/*.json

Each cache entry has URL, hash, and timestamp. Purging should be possible either for the whole cache, by individual hash, or by individual URL. Listing should be possible to limit by URL url=, or base URL url=^ or by timestamp (date=, date=<, date=>).