sources: factor out Source::URL::Plurk.

Also fix it so that for adult works, we get the images posted by the
artist in the replies. Example: https://www.plurk.com/p/omc64y (nsfw).

app/logical/source/url.rb

@@ -19,6 +19,7 @@ module Source
   class URL < Danbooru::URL
     SUBCLASSES = [
       Source::URL::Twitter,
+      Source::URL::Plurk,
       Source::URL::Skeb,
       Source::URL::TwitPic,
       Source::URL::Foundation,