[Feature Request] Make ActionController's middleware stack configurable per-controller basis

Right now `--api` is all or nothing. Most applications that have an API also have a companion web site. If you plan to keep separated applications, that's fine. The problem lies on using the same app for both your web site and API. My proposal is using whatever is defined as the `middleware_stack` on the controller, without injecting/merging any default behavior. In practice, we would set `ActionController::Base.middleware_stack` to `Rails::Application::DefaultMiddlewareStack.new` on railties, allowing replacement per controller basis, like the following:
class ActionController::Base
self.middleware_stack = Rails::Application::DefaultMiddlewareStack.new
class ApplicationController < ActionController::Base
class ApiController < ActionController::Base
# This would override the super class (ActionController::Base) middleware stack.
self.middleware_stack = ActionController::MiddlewareStack.new
# This controller would have just the following middleware.
use ::Rack::Runtime

I’m glad to hear your suggestions. I can happily jump on this.