Add helper methods that will be accessible from any endpoint within this namespace (and child namespaces).

When called without a block, all known helpers within this scope are included.

@param [Array] new_modules optional array of modules to include @param [Block] block optional block of methods to include

@example Define some helpers.

    class ExampleAPI < Grape::API
      helpers do
        def current_user
          User.find_by_id(params[:token])
        end
      end
    end

@example Include many modules

    class ExampleAPI < Grape::API
      helpers Authentication, Mailer, OtherModule
    end