implement 'image-unique' and 'chapter-unique' options (#303)

The default value for both is 'false', i.e. duplicate URLs are NOT
ignored.

The previous behavior was to always ignore duplicate URLs to make
'--abort-on-skip' work properly when new images where added to the
beginning of a collection while gallery-dl is running.

docs/configuration.rst

@@ -381,6 +381,28 @@ Description Like `image-filter`__, but applies to delegated URLs
 __ `extractor.*.image-filter`_
 
 
+extractor.*.image-unique
+------------------------
+=========== =====
+Type        ``bool``
+Default     ``false``
+Description Ignore image URLs that have been encountered before during the
+            current extractor run.
+=========== =====
+
+
+extractor.*.chapter-unique
+--------------------------
+=========== =====
+Type        ``bool``
+Default     ``false``
+Description Like `image-unique`__, but applies to delegated URLs
+            like manga-chapters, etc.
+=========== =====
+
+__ `extractor.*.image-unique`_
+
+
 
 Extractor-specific Options
 ==========================