store grenaded tabs in bookmarks folder

[sunaku]

Hello,

Please use bookmarks as the backing store for grenaded tabs because Firefox automatically backs them up. 🚀 This will also eliminate issues like #12, #13, and #29 where people lose their backing store (and thus all their grenaded tabs) randomly and due to no fault of their own. 😱

For inspiration, take a look at the Sesh extension for Google Chrome, which does this beautifully. 💖

Thanks for your consideration.