added zoom in & zoom out

lib/main-menu.js

@@ -266,6 +266,13 @@ const view = {
       click () {
         mainWindow.setFullScreen(!mainWindow.isFullScreen())
       }
+    },
+    {
+      role: 'zoomin',
+      accelerator: macOS ? 'CommandOrControl+Plus' : 'Control+='
+    },
+    {
+      role: 'zoomout'
     }
   ]
 }