[twitter] changes to 'cards' option

- change default value to 'true'
- only invoke youtube-dl for cards unsupported by gallery
  when 'cards' is set to "ytdl"

"cards": true   --> only download card images
"cards": "ytdl" --> download card images and
                    use youtube_dl on otherwise unsupported cards

gallery_dl/extractor/twitter.py

@@ -39,7 +39,7 @@ class TwitterExtractor(Extractor):
         self.pinned = self.config("pinned", False)
         self.quoted = self.config("quoted", False)
         self.videos = self.config("videos", True)
-        self.cards = self.config("cards", False)
+        self.cards = self.config("cards", True)
         self._user_cache = {}
         self._init_sizes()
 
@@ -165,7 +165,7 @@ class TwitterExtractor(Extractor):
                     tweet, data["media_entities"].values(), files)
                 return
 
-        if self.videos:
+        if self.cards == "ytdl":
             url = "ytdl:{}/i/web/status/{}".format(self.root, tweet["id_str"])
             files.append({"url": url})