* All remotes now support RangeOption so remove SeekOption
* Correct off by one error as RangeOption arguments are inclusive.
* Use RangeSeek in preference to Seek if available
A Range request can never request 0 bytes however this change was made
to make a clearer signal that the limit means read to the end.
Add test and more documentation and fixup uses
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.