def get_plugin(name)
      raise "Unknown plugin: #{ name }" unless Capistrano::EXTENSIONS.keys.include?(name)
      self.send(name)
    end
def get_plugin(name)
      raise "Unknown plugin: #{ name }" unless Capistrano::EXTENSIONS.keys.include?(name)
      self.send(name)
    end
given a plugin name, return the plugin.