[pixiv] update (#1304)

- remove login with username & password
- require a refresh token
- add 'oauth:pixiv' functionality

See also:
- https://github.com/upbit/pixivpy/issues/158
- https://gist.github.com/ZipFile/c9ebedb224406f4f11845ab700124362

scripts/supportedsites.py

@@ -189,7 +189,7 @@ AUTH_MAP = {
     "patreon"        : _COOKIES,
     "pawoo"          : _OAUTH,
     "pinterest"      : "Supported",
-    "pixiv"          : "Required",
+    "pixiv"          : _OAUTH,
     "reddit"         : _OAUTH,
     "sankaku"        : "Supported",
     "seiga"          : "Required",