[twitter] add extractor for media-tweet timelines (#96)

For example "https://twitter.com/PicturesEarth/media".
They are different from normal timelines in that they do not contain
any (re)tweets from other users and feature all media the user ever
posted, including responses to other tweets.

scripts/build_supportedsites.py

@@ -68,6 +68,7 @@ SUBCATEGORY_MAP = {
     "issue"  : "Comic-Issues",
     "manga"  : "Manga",
     "me"     : "pixiv.me Links",
+    "media"  : "Media Tweets",
     "path"   : "Images from Users and Folders",
     "pinit"  : "pin.it Links",
     "popular": "Popular Images",
@@ -226,6 +227,8 @@ def category_key(extrlist):
 def subcategory_key(cls):
     if cls.subcategory in ("user", "issue"):
         return "A"
+    if cls.subcategory in ("media",):
+        return "z"
     return cls.subcategory