typings: add app to app.use cb (#665)

Author: seangenabe

index.d.ts

@@ -6,7 +6,7 @@ export = Choo
 
 declare class Choo {
   constructor (opts?: Choo.IChoo)
-  use (callback: (state: Choo.IState, emitter: EventEmitter) => void): void
+  use (callback: (state: Choo.IState, emitter: EventEmitter, app: this) => void): void
   route (routeName: string, handler: (state: Choo.IState, emit: (name: string, ...args: any[]) => void) => void): void
   mount (selector: string): void
   start (): HTMLElement