add linkExactActiveClass to router options

From version 2.5.0+ exact active class supported https://router.vuejs.org/en/api/options.html#linkexactactiveclass

lib/nuxt.js

@@ -37,6 +37,7 @@ class Nuxt {
         base: '/',
         middleware: [],
         linkActiveClass: 'nuxt-link-active',
+        linkExactActiveClass: 'nuxt-link-exact-active',
         extendRoutes: null,
         scrollBehavior: null
       },