Auto backup data in Google Drive

[shuowu]

Background:

Currently YiNote only storage notes data in the browser's local storage, which may get lost by accident due to uninstallation or bugs introduced with a new release.

Feature request:

• YiNote should be able to periodically sync notes data to users' cloud storage, like Google Drive.
• Sync should happen once a day.
• Cloud storages to support: Google drive.
• User should be able to turn on auto backup in options page.