[twitter] add experimental 'videos' option (#99)

Enabling this option will detect videos in tweets and output them as
"unsupported" URLs, so that these can then be downloaded with youtube-dl

There are a lot of improvements to be made to the current
implementation, but it works and does what it is supposed to, even if
inefficient as can be ...

docs/gallery-dl.conf

@@ -110,7 +110,8 @@
         },
         "twitter":
         {
-            "retweets": true
+            "retweets": true,
+            "videos": false
         },
         "booru":
         {