Clarifications on cover art downloading

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• For a case like redownload cover.jpg in better quality #260 it would be nice to have a switch --covers-only which downloads and optionally embeds covers but does not download songs
• Embedding is unclear - does it embed instead of saving the coverfile? Into every single song for an album? Does it work at all? Does it embed the higher quality cover when using --og-cover?

As mentioned in #126, more fine-grained control would be helpful.
I personally would like to download high-quality coverfiles for albums, and embed low-quality images for singles - this seems to me a very reasonable tradeoff of completeness, size and quality.