[twitter] support specifying users by ID (#980)

by using 'id:…' as their screen name, i.e.
https://www.twitter.com/id:2976459548/media
instead of
https://twitter.com/supernaturepics/media

The user ID can, for example, be obtained from the output of
$ gallery-dl -j --range 1 https://twitter.com/<screen-name>

gallery_dl/extractor/twitter.py

@@ -252,6 +252,7 @@ class TwitterTimelineExtractor(TwitterExtractor):
             "url": "0106229d408f4111d9a52c8fd2ad687f64842aa4",
         }),
         ("https://mobile.twitter.com/supernaturepics?p=i"),
+        ("https://www.twitter.com/id:2976459548"),
     )
 
     def tweets(self):
@@ -268,6 +269,7 @@ class TwitterMediaExtractor(TwitterExtractor):
             "url": "0106229d408f4111d9a52c8fd2ad687f64842aa4",
         }),
         ("https://mobile.twitter.com/supernaturepics/media#t"),
+        ("https://www.twitter.com/id:2976459548/media"),
     )
 
     def tweets(self):
@@ -450,18 +452,18 @@ class TwitterAPI():
         return tweets
 
     def timeline_profile(self, screen_name):
-        user = self.user_by_screen_name(screen_name)
-        endpoint = "2/timeline/profile/{}.json".format(user["rest_id"])
+        user_id = self._user_id_by_screen_name(screen_name)
+        endpoint = "2/timeline/profile/{}.json".format(user_id)
         return self._pagination(endpoint)
 
     def timeline_media(self, screen_name):
-        user = self.user_by_screen_name(screen_name)
-        endpoint = "2/timeline/media/{}.json".format(user["rest_id"])
+        user_id = self._user_id_by_screen_name(screen_name)
+        endpoint = "2/timeline/media/{}.json".format(user_id)
         return self._pagination(endpoint)
 
     def timeline_favorites(self, screen_name):
-        user = self.user_by_screen_name(screen_name)
-        endpoint = "2/timeline/favorites/{}.json".format(user["rest_id"])
+        user_id = self._user_id_by_screen_name(screen_name)
+        endpoint = "2/timeline/favorites/{}.json".format(user_id)
         return self._pagination(endpoint)
 
     def timeline_bookmark(self):
@@ -490,6 +492,11 @@ class TwitterAPI():
         except KeyError:
             raise exception.NotFoundError("user")
 
+    def _user_id_by_screen_name(self, screen_name):
+        if screen_name.startswith("id:"):
+            return screen_name[3:]
+        return self.user_by_screen_name(screen_name)["rest_id"]
+
     @cache(maxage=3600)
     def _guest_token(self):
         endpoint = "1.1/guest/activate.json"