add '--cbz' command-line option

docs/options.md

@@ -119,6 +119,7 @@
 
 ## Post-processing Options:
     --zip                       Store downloaded files in a ZIP archive
+    --cbz                       Store downloaded files in a CBZ archive
     --ugoira-conv               Convert Pixiv Ugoira to WebM (requires FFmpeg)
     --ugoira-conv-lossless      Convert Pixiv Ugoira to WebM in VP9 lossless
                                 mode

gallery_dl/option.py

@@ -468,6 +468,15 @@ def build_parser():
         action="append_const", const="zip",
         help="Store downloaded files in a ZIP archive",
     )
+    postprocessor.add_argument(
+        "--cbz",
+        dest="postprocessors",
+        action="append_const", const={
+            "name"     : "zip",
+            "extension": "cbz",
+        },
+        help="Store downloaded files in a CBZ archive",
+    )
     postprocessor.add_argument(
         "--ugoira-conv",
         dest="postprocessors", action="append_const", const={