Enhancement: Allow using rclone to access other services

We've added the ability to use rclone to store backup data on all backends that
it supports. This was done in collaboration with Nick, the author of rclone.
You can now use it to first configure a service, then restic manages the rest
(starting and stopping rclone). For details, please see the manual.

https://github.com/restic/restic/issues/1561
https://github.com/restic/restic/pull/1657
https://rclone.org