[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/gallery-dl.conf

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