Support fetching cover art images from the Cover Art Archive
Add option `--cover-art` to `whipper cd rip` command which accepts three values:
- `file`: save the downloaded cover image as standalone file in the rip folder (named `cover.jpg`)
- `embed`: embed the download cover image into all the ripped audio tracks (no standalone file will be kept)
- `complete`: save standalone cover image as standalone file and embed it into all the ripped audio tracks (`file` + `embed`)
Every cover art is fetched from the Cover Art Archive as JPEG thumbnail with a maximum dimension of 500px.
Other supported values for the thumbnails are 250, 500 and 1200 (currently only some images have a corresponding 1200px sized thumbnail).
This feature introduces an optional dependency on the `Pillow` module which is required for the decoding of the cover file (required by the `embed` and `complete` option values).