Add option to not save thumbnails/images

[columndeeply]

Could a "Don't download images" option be added? I'm using shiori in list mode + hidden thumbnails, but I still end up with a bunch of images downloaded. The download code is here:

shiori/internal/core/processing.go

Lines 127 to 144 in 40ae4cd

	for i, imageURL := range imageURLs {
	err = DownloadBookImage(deps, imageURL, imgPath)
	if err != nil && errors.Is(err, ErrNoSupportedImageType) {
	log.Printf("%s: %s", err, imageURL)
	if i == len(imageURLs)-1 {
	deps.Domains.Storage.FS().Remove(imgPath)
	}
	}
	if err != nil {
	log.Printf("File download not successful for image URL: %s", imageURL)
	continue
	}
	if err == nil {
	book.ImageURL = fp.Join("/", "bookmark", strID, "thumb")
	book.ModifiedAt = ""
	break
	}
	}

I've tried removing that chunk of code and recompiling and it works as intended, new bookmarks don't download any images. I'm guessing wrapping that loop in an "if" condition (like the book.CreateEbook in that same file) should work.

If this sounds good to you I can try to write a pull request with these changes.