Add rake task for Banzai rendering

lib/tasks/gitlab/banzai.rake

+# frozen_string_literal: true
+
+namespace :gitlab do
+  namespace :banzai do
+    desc 'GitLab | Banzai | Render markdown using our FullPipeline (input will be requested)'
+    task :render => :environment do |_t|
+      markdown = Array.new
+
+      puts "\nEnter markdown below, Ctrl-D to end (if you need blank lines, paste in the full text):"
+      while buf = Readline.readline('', true) do
+	      markdown << buf
+      end
+
+      puts "Rendering using Gitlab's FullPipeline...\n\n"
+      puts Banzai::Pipeline::FullPipeline.to_html(markdown.join("\n"), project: nil)
+    end
+  end
+end