add static 404.html

lib/builder/generator.js

@@ -109,6 +109,7 @@ export default class Generator extends Tapable {
           }
         }
         let path = join(route, sep, 'index.html') // /about -> /about/index.html
+        path = (path === '/404/index.html') ? '/404.html' : path // /404 -> /404.html
         debug('Generate file: ' + path)
         path = join(distPath, path)
         // Make sure the sub folders are created