FR: add setting sync support

[dixi83]

Hey, love your addon in FF! is it possidble to add sync support for settings?

https://wiki.mozilla.org/Services/Sync/Addon_Sync#Are_Add-on_Settings_Synchronized.3F
https://developer.mozilla.org/en-US/search?q=Syncing%20custom%20preferences

[jonerrr]

Thank you for the idea! This should be possible but ATM I sadly don't have the time to add it.