feature(core): allow for multiple view dirs in Express

In Express, you can set a string or an array of string as
base view paths. The views are looked up in the order they occur in
the array. The ExpressAdapter only allowed for a single string, now it
accepts an array of strings as well.

closes nestjs#1513

Author: kiwikern

packages/common/interfaces/http/http-server.interface.ts

@@ -38,7 +38,7 @@ export interface HttpServer {
   setErrorHandler?(handler: Function);
   setNotFoundHandler?(handler: Function);
   useStaticAssets?(...args: any[]): this;
-  setBaseViewsDir?(path: string): this;
+  setBaseViewsDir?(path: string | string[]): this;
   setViewEngine?(engineOrOptions: any): this;
   createMiddlewareFactory(
     method: RequestMethod,

packages/common/interfaces/nest-express-application.interface.ts

@@ -43,12 +43,12 @@ export interface INestExpressApplication {
   useStaticAssets(path: string, options?: ServeStaticOptions): this;
 
   /**
-   * Sets a base directory for templates (views).
+   * Sets one or multiple base directories for templates (views).
    * Example `app.setBaseViewsDir('views')`
    *
    * @returns {this}
    */
-  setBaseViewsDir(path: string): this;
+  setBaseViewsDir(path: string | string[]): this;
 
   /**
    * Sets a view engine for templates (views).

packages/core/adapters/express-adapter.ts

@@ -126,7 +126,7 @@ export class ExpressAdapter implements HttpServer {
     return this.use(express.static(path, options));
   }
 
-  setBaseViewsDir(path: string) {
+  setBaseViewsDir(path: string | string[]) {
     return this.set('views', path);
   }
 

packages/core/nest-application.ts

@@ -363,7 +363,7 @@ export class NestApplication extends NestApplicationContext
     return this;
   }
 
-  public setBaseViewsDir(path: string): this {
+  public setBaseViewsDir(path: string | string[]): this {
     this.httpAdapter.setBaseViewsDir && this.httpAdapter.setBaseViewsDir(path);
     return this;
   }