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,

app/logical/source/url/plurk.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+# Notes
+#
+# * Posts can have up to 10 images.
+# * Artists commonly post extra images by replying to their own post.
+# * Adult posts are hidden for logged out users. The main images can be found by
+#   scraping a <script> tag, but an API call is needed to get the images in the replies.
+#
+# Image URLs
+#
+# * https://images.plurk.com/5wj6WD0r6y4rLN0DL3sqag.jpg
+#
+# Thumbnail URLs
+#
+# * https://images.plurk.com/mx_5wj6WD0r6y4rLN0DL3sqag.jpg
+#
+# Page URLs
+#
+# * https://www.plurk.com/p/om6zv4 (non-adult, single image)
+# * https://www.plurk.com/p/okxzae (non-adult, multiple images, with replies)
+# * https://www.plurk.com/p/omc64y (adult, multiple images, with replies)
+# * https://www.plurk.com/m/p/omc64y
+#
+# Profile URLs
+#
+# * https://www.plurk.com/redeyehare
+# * https://www.plurk.com/m/redeyehare
+
+class Source::URL::Plurk < Source::URL
+  attr_reader :username, :work_id
+
+  def self.match?(url)
+    url.domain == "plurk.com"
+  end
+
+  def parse
+    case [domain, *path_segments]
+
+    # https://images.plurk.com/5wj6WD0r6y4rLN0DL3sqag.jpg
+    # https://images.plurk.com/mx_5wj6WD0r6y4rLN0DL3sqag.jpg
+    in "plurk.com", /^(mx_)?(\w{22})\.(\w+)$/
+      @filename, @file_ext = $2, $3
+
+    # https://www.plurk.com/p/om6zv4
+    in "plurk.com", "p", work_id
+      @work_id = work_id
+
+    # https://www.plurk.com/m/p/okxzae
+    in "plurk.com", "m", "p", work_id
+      @work_id = work_id
+
+    # https://www.plurk.com/redeyehare
+    in "plurk.com", username
+      @username = username
+
+    # https://www.plurk.com/m/redeyehare
+    in "plurk.com", "m", username
+      @username = username
+
+    else
+    end
+  end
+
+  def image_url?
+    host == "images.plurk.com"
+  end
+end