This unifies the 3 methods of reading config
* command line
* environment variable
* config file
And allows them all to be configured in all places. This is done by
making the []fs.Option in the backend registration be the master
source of what the backend options are.
The backend changes are:
* Use the new configmap.Mapper parameter
* Use configstruct to parse it into an Options struct
* Add all config to []fs.Option including defaults and help
* Remove all uses of pflag
* Remove all uses of config.FileGet
Very large directories can have their sizes returned as floating point
numbers, eg `1.0034576985781e+14` from the box API.
Before this change this would fail to parse as an int64.
This change parses the size as a float64 instead which will be
perfectly accurate for sizes up to 2**56 which is about 9 PB.
It is unknown whether box themselves use a float64 as an intermediate
representation in the API or not - it seems likely.
Fixes#2261
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.
