Add task to prepare releasing

This task updated Changelog, version.rb, and Gemfile.lock.

lib/tasks/release.rake

@@ -0,0 +1,49 @@
+namespace :release do
+  desc "Add header of now version release to Changelog and bump up version"
+  task :prepare do
+    raise "Use this task in development only" unless Rails.env.development?
+
+    # detect merged PR
+    now_version = FluentdUI::VERSION
+    pr_numbers = `git log v#{now_version}..master --oneline`.scan(/#[0-9]+/)
+
+    if !$?.success? || pr_numbers.empty?
+      puts "Detecting PR failed. Please confirm if any PR were merged after the latest release."
+      exit(false)
+    end
+
+    # Generate new version
+    /\.([0-9]+)\z/.match(now_version)
+    now_revision = $1
+    new_version = now_version.gsub(/\.#{now_revision}\z/, ".#{now_revision.to_i + 1}")
+
+    # Update Changelog
+    changelog_filename = Rails.root.join('Changelog')
+    changelog = File.read(changelog_filename)
+
+    pr_descriptions = pr_numbers.map do |number|
+      "* [] #{number} https://github.com/fluent/fluentd-ui/pull/#{number.gsub('#', '')}"
+    end.join("\n")
+
+    new_changelog = <<-HEADER
+Release #{new_version} - #{Time.now.strftime("%Y/%m/%d")}
+#{pr_descriptions}
+
+#{changelog.chomp}
+HEADER
+
+    File.open(changelog_filename, "w") {|f| f.write(new_changelog)}
+
+    # Update version.rb
+    version_filename = Rails.root.join("lib", "fluentd-ui", "version.rb")
+    version_class = File.read(version_filename)
+    new_version_class = version_class.gsub(/VERSION = \"#{now_version}\"/, "VERSION = \"#{new_version}\"")
+
+    File.open(version_filename, 'w') {|f| f.write(new_version_class)}
+
+    # Update Gemfile.lock
+    system("bundle install")
+
+    puts 'Changelog, verion and Gemfile.lock is updated. New version is #{new_version}.'
+  end
+end