Add generator for plugins

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

generator_templates/plugins/template.rb

+# Requirements
+# * File name must end with _s.rb. For example, jenkins_plugin.rb.
+# * All code should be inside class. No code should be executed on file load.
+# * Class name must be same as file name.
+#   If file name is jenkins_plugin.rb then class name must be JenkinsPlugin.
+#
+# Reccomendations
+# * Code should not depend on or use GitLab classes and other code.
+# * Consider contributing your plugin to GitLab source code so we can test it
+#   and make sure it will work in further version.
+#
+class $NAMEPlugin
+  def execute(data)
+    # TODO: Implement me
+  end
+end

lib/tasks/plugins.rake

+namespace :plugins do
+  desc 'Generate skeleton for new plugin'
+  task generate: :environment do
+    ARGV.each { |a| task a.to_sym { } }
+    name = ARGV[1]
+
+    unless name.present?
+      puts 'Error. You need to specify a name for the plugin'
+      exit 1
+    end
+
+    class_name = name.classify
+    param = name.underscore
+    file_path = Rails.root.join('plugins', param + '_plugin.rb')
+    template = File.read(Rails.root.join('generator_templates', 'plugins', 'template.rb'))
+    template.gsub!('$NAME', class_name)
+
+    if File.write(file_path, template)
+      puts "Done. Your plugin saved under #{file_path}."
+      puts 'Feel free to edit it.'
+    else
+      puts "Failed to save #{file_path}."
+    end
+  end
+end