Add context propagation to rclone

- Change rclone/fs interfaces to accept context.Context
- Update interface implementations to use context.Context
- Change top level usage to propagate context to lover level functions

Context propagation is needed for stopping transfers and passing other
request-scoped values.

cmd/copyurl/copyurl.go

@@ -1,6 +1,8 @@
 package copyurl
 
 import (
+	"context"
+
 	"github.com/ncw/rclone/cmd"
 	"github.com/ncw/rclone/fs/operations"
 	"github.com/spf13/cobra"
@@ -22,7 +24,7 @@ without saving it in tmp storage.
 		fsdst, dstFileName := cmd.NewFsDstFile(args[1:])
 
 		cmd.Run(true, true, command, func() error {
-			_, err := operations.CopyURL(fsdst, dstFileName, args[0])
+			_, err := operations.CopyURL(context.Background(), fsdst, dstFileName, args[0])
 			return err
 		})
 	},