add command shortcuts & menu command

public/_locales/en/messages.json

@@ -4,5 +4,8 @@
   },
   "app_description": {
     "message": "A minimalist bilingual translation Extension & Greasemonkey Script"
+  },
+  "toggle_translate": {
+    "message": "Toggle Translate"
   }
 }

public/_locales/zh_CN/messages.json

@@ -4,5 +4,8 @@
   },
   "app_description": {
     "message": "一个简约的双语网页翻译扩展 & 油猴脚本"
+  },
+  "toggle_translate": {
+    "message": "切换翻译"
   }
 }