* Use Indexeddb to save URL list in service worker
* Improve storage organisation (breaks compatibility)
* Provide Diff Tool for collaboration
* Communication between tabs:
  - https://gist.github.com/neilj/4146038
  - http://nmjenkins.com/presentations/intertab-communication.html#/12
  - https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker
  - https://github.com/tejacques/crosstab
