chore($theme-default): using search plugin

Author: ulivz

Diff for: packages/@vuepress/theme-default/index.js

@@ -4,6 +4,7 @@ const themePlugin = require('./plugin')
 module.exports = {
   plugins: [
     themePlugin,
-    '@vuepress/active-header-links'
+    '@vuepress/active-header-links',
+    '@vuepress/search'
   ]
 }

Diff for: packages/@vuepress/theme-default/package.json

@@ -24,9 +24,10 @@
   "homepage": "https://github.com/vuejs/vuepress/packages/@vuepress/theme-default#readme",
   "dependencies": {
     "@vuepress/plugin-active-header-links": "1.0.0",
+    "@vuepress/plugin-search": "1.0.0",
     "docsearch.js": "^2.5.2",
     "nprogress": "^0.2.0",
     "stylus": "^0.54.5",
     "stylus-loader": "^3.0.2"
   }
-}
+}