[sankaku] rewrite/improve (fixes #44)

- add wait-time between HTTP requests similar to exhentai
- add 'wait-min' and 'wait-max' options
- increase retry-count for HTTP requests to 10
- implement user authentication (non-authenticated users can only view
  images up to page 25)
- implement 'skip()' functionality (only works up to page 50)
- implement image-retrieval for pages >= 51
- fix issue with multiple tags

docs/configuration.rst

@@ -238,7 +238,8 @@ Description The username and password to use when attempting to log in to
 
             Specifying username and password is
             required for the ``pixiv``, ``nijie`` and ``seiga`` modules and
-            optional (but strongly recommended) for ``batoto`` and ``exhentai``.
+            optional (but strongly recommended) for ``batoto``, ``exhentai``
+            and ``sankaku``.
 
             These values can also be set via the ``-u/--username`` and
             ``-p/--password`` command-line options or by using a |.netrc|_ file.
@@ -522,6 +523,20 @@ Description The ``refresh_token`` value you get from linking your Reddit account
 =========== =====
 
 
+extractor.sankaku.wait-min & .wait-max
+--------------------------------------
+=========== =====
+Type        ``float``
+Default     ``2.0`` and ``4.0``
+Description Minimum and maximum wait time in seconds between each image
+
+            Sankaku Channel responds with ``429 Too Many Requests`` if it
+            receives too many HTTP requests in a certain amount of time.
+            Waiting a few seconds between each request tries to prevent that.
+=========== =====
+
+
+
 API Tokens & IDs
 ================
 

docs/gallery-dl.conf

@@ -49,6 +49,11 @@
             "username": null,
             "password": null
         },
+        "sankaku":
+        {
+            "username": null,
+            "password": null
+        },
         "seiga":
         {
             "username": null,

docs/supportedsites.rst

@@ -63,7 +63,7 @@ RebeccaBlackTech     https://rbt.asia/                   Threads
 Reddit               https://reddit.com/                 Submissions, Subreddits                            Optional (OAuth)
 Rule 34              https://rule34.xxx/                 Posts, Tag-Searches
 Safebooru            https://safebooru.org/              Posts, Tag-Searches
-Sankaku Channel      https://chan.sankakucomplex.com/    Tag-Searches
+Sankaku Channel      https://chan.sankakucomplex.com/    Tag-Searches                                       Optional
 Sea Otter Scans      https://reader.seaotterscans.com/   Chapters, Manga
 Sen Manga            http://raw.senmanga.com/            Chapters
 Sense-Scans          http://sensescans.com/              Chapters, Manga