Add rclone support

[sorgfresser]

I am currently investigating what it would take to add support for rclone as a potential way to address #354
So far, an initial plan of mine would consist of

• Adding a new BackupDestination and implementing its methods (using https://github.com/rclone/rclone/blob/master/librclone/python/rclone.py)
• Build a new FolderFinder (though I am unsure whether rclone really does the right level of abstraction here, maybe we still need some provider specific requests
• Create a custom version of AuthCodeQuery for rclone
• Adding new exceptions on the way as needed
• Provide these new things and incorporate them as alternatives (that's the part where I would be very glad if you could point me in the correct direction)

Since there is quite a lot of things to be done and I am sure I forgot something, I would greatly appreciate any remarks on things I missed out on or how to achieve the things named above.
Additionally I wanted to discuss this change first as remarked in the CONTRIBUTING.md