feat(d.ts): add middleware to Application and other ts improvement (#…

…2465)

* add middleware to Application
* plugin type can be boolean
* rename EggPluginValue to EggPluginItem

index.d.ts

@@ -605,6 +605,8 @@ declare module 'egg' {
     controller: IController;
 
     Controller: Controller;
+
+    middleware: KoaApplication.Middleware[] & IMiddleware;
   }
 
   export interface FileStream extends Readable { // tslint:disable-line
@@ -878,11 +880,13 @@ declare module 'egg' {
    *
    * Now I can get ctx.service.foo at controller and other service file.
    */
-  export interface IService { } // tslint:disable-line
+  export interface IService extends PlainObject { } // tslint:disable-line
+
+  export interface IController extends PlainObject { } // tslint:disable-line
 
-  export interface IController { } // tslint:disable-line
+  export interface IMiddleware extends PlainObject {} // tslint:disable-line
 
-  export interface IHelper {
+  export interface IHelper extends PlainObject {
     /**
      * Generate URL path(without host) for route. Takes the route name and a map of named params.
      * @method Helper#pathFor
@@ -919,15 +923,17 @@ declare module 'egg' {
   export type EggEnvType = 'local' | 'unittest' | 'prod' | string;
 
   /**
-   * plugin config item
+   * plugin config item interface
    */
-  export interface EggPluginItem {
+  export interface IEggPluginItem {
     env?: EggEnvType[];
     path?: string;
     package?: string;
     enable?: boolean;
   }
 
+  export type EggPluginItem = IEggPluginItem | boolean;
+
   /**
    * build-in plugin list
    */