breaking changes: Open_Graph_meta 設置更改，可配置其它參數

feature: 更新 Facebook sdk 到 v15
improvement: 更新 plugins.yml
Remove: 移除 meta keywords

layout/includes/third-party/chat/messenger.pug

@@ -9,7 +9,7 @@ script.
   window.fbAsyncInit = function() {
     FB.init({
       xfbml: true,
-      version: 'v14.0'
+      version: 'v15.0'
     });
   };