[exhentai] add 'source' option

setting it to "hitomi" downloads the corresponding gallery from
hitomi.la; might be extended to other sources in the future

docs/configuration.rst

@@ -1081,17 +1081,6 @@ Description
     Minimum wait time in seconds before API requests.
 
 
-extractor.exhentai.limits
--------------------------
-Type
-    ``integer``
-Default
-    ``null``
-Description
-    Sets a custom image download limit and
-    stops extraction when it gets exceeded.
-
-
 extractor.exhentai.domain
 -------------------------
 Type
@@ -1105,6 +1094,17 @@ Description
     * ``"exhentai.org"``: Use ``exhentai.org`` for all URLs
 
 
+extractor.exhentai.limits
+-------------------------
+Type
+    ``integer``
+Default
+    ``null``
+Description
+    Sets a custom image download limit and
+    stops extraction when it gets exceeded.
+
+
 extractor.exhentai.metadata
 ---------------------------
 Type
@@ -1129,6 +1129,18 @@ Description
     Download full-sized original images if available.
 
 
+extractor.exhentai.source
+-------------------------
+Type
+    ``string``
+Default
+    ``"gallery"``
+Description
+    Selects an alternative source to download files from.
+
+    * ``"hitomi"``:  Download the corresponding gallery from ``hitomi.la``
+
+
 extractor.fanbox.embeds
 -----------------------
 Type