Issue: Scraper not limited by license

[winderine]

At least one of the sounds on the soundtrack generator was in fact licensed under a CC-BY-NC rather than a CC-0 license, but there was no attribution available. Providing attribution as I requested in #94 would solve the problem for that particular sound file, but what if CC-ND files are getting used as well? Some kind of license picker or assurance that only public domain sounds are being used would provide confidence.