Implement new backend config system

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

rclone.go

@@ -4,8 +4,6 @@
 package main
 
 import (
-	"log"
-
 	"github.com/ncw/rclone/cmd"
 
 	_ "github.com/ncw/rclone/backend/all" // import all backends
@@ -13,7 +11,5 @@ import (
 )
 
 func main() {
-	if err := cmd.Root.Execute(); err != nil {
-		log.Fatalf("Fatal error: %v", err)
-	}
+	cmd.Main()
 }