Add an UseRouter that takes Action<IRouteBuilder>

- Added an overload of UseRouter that takes Action<IRouteBuilder>, to
make configuring standalone routing much less verbose.

Addresses #332

src/Microsoft.AspNetCore.Routing/BuilderExtensions.cs

@@ -41,5 +41,30 @@ public static IApplicationBuilder UseRouter(this IApplicationBuilder builder, IR
 
             return builder.UseMiddleware<RouterMiddleware>(router);
         }
+
+        /// <summary>
+        /// Adds a <see cref="RouterMiddleware"/> middleware to the specified <see cref="IApplicationBuilder"/>
+        /// with the <see cref="IRouter"/> built from configured <see cref="IRouteBuilder"/>.
+        /// </summary>
+        /// <param name="builder">The <see cref="IApplicationBuilder"/> to add the middleware to.</param>
+        /// <param name="action">An <see cref="Action{IRouteBuilder}"/> to configure the provided <see cref="IRouteBuilder"/>.</param>
+        /// <returns>A reference to this instance after the operation has completed.</returns>
+        public static IApplicationBuilder UseRouter(this IApplicationBuilder builder, Action<IRouteBuilder> action)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            if (action == null)
+            {
+                throw new ArgumentNullException(nameof(action));
+            }
+
+            var routeBuilder = new RouteBuilder(builder);
+            action(routeBuilder);
+
+            return builder.UseRouter(routeBuilder.Build());
+        }
     }
 }