support no fallback on browsers which do not support history API (vuejs#1512)

Author: JounQin

flow/declarations.js

@@ -30,6 +30,7 @@ declare type AfterNavigationHook = (to: Route, from: Route) => any
 declare type RouterOptions = {
   routes?: Array<RouteConfig>;
   mode?: string;
+  fallback?: boolean;
   base?: string;
   linkActiveClass?: string;
   parseQuery?: (query: string) => Object;

src/index.js

@@ -42,7 +42,7 @@ export default class VueRouter {
     this.matcher = createMatcher(options.routes || [], this)
 
     let mode = options.mode || 'hash'
-    this.fallback = mode === 'history' && !supportsPushState
+    this.fallback = mode === 'history' && !supportsPushState && options.fallback !== false
     if (this.fallback) {
       mode = 'hash'
     }