(concord-images-add-url): Add optional arguments `prefix' and `prefer-iiif'.
(concord-images-encode-url-as-id): Add optional argument `base'. (concord-images-add-url): Likewise. (concord-images-add-iiif): Likewise.
(concord-images-encode-url-as-id): Support image.chise.org and image.kanji.zinbun.kyoto-u.ac.jp.
(concord-images-add-url): Use feature `image-width' and feature `image-height' instead of feature `width' and feature `height'. (concord-images-add-segments): Likewise; use feature `image-offset-x' and feature `image-offset-y' instead of `offset-x' and `offset-y'.
New file.