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

_config.yml

@@ -857,7 +857,16 @@ lazyload:
 
 # Open graph meta tags
 # https://developers.facebook.com/docs/sharing/webmasters/
-Open_Graph_meta: true
+Open_Graph_meta:
+  enable: true
+  option:
+    # twitter_card:
+    # twitter_image:
+    # twitter_id:
+    # twitter_site:
+    # google_plus:
+    # fb_admins:
+    # fb_app_id:
 
 # Add the vendor prefixes to ensure compatibility
 css_prefix: true