Try to limit generated atom names to the / directory

This is necessary in consequence of issue #2.

handler_pipe.go

@@ -3,6 +3,7 @@ package tabacco
 import (
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -126,7 +127,7 @@ func expandVars(s string, backup Backup, ds Dataset, atom Atom) string {
 
 func makeSingleAtomDataset(ds Dataset, atom Atom) Dataset {
 	return Dataset{
-		Name:    filepath.Join(ds.Name, atom.Name),
+		Name:    fmt.Sprintf("/%s.%s", ds.Name, atom.Name),
 		Handler: ds.Handler,
 		Atoms:   []Atom{atom},
 	}