Try Jekyll Hook for 3rd party publishing

_plugins/publish_to_medium.rb

@@ -0,0 +1,13 @@
+require 'jekyll'
+
+Jekyll::Hooks.register :documents, :pre_render do |post|
+  if PublishToMedium.suitable?(post)
+    p post.data["slug"]
+  end
+end
+
+class PublishToMedium
+  def self.suitable?(post)
+    post.data["categories"].include?("tumblelog") && ["text", nil].include?(post.data["type"])
+  end
+end