fix #80: using os.MkdirAll():

Recursively create ad dir including output dir. The output dir itself
is not being created separately anymore. That way, no directory will
be created if no ads could be downloaded.

util.go

@@ -32,7 +32,7 @@ import (
 
 func Mkdir(dir string) error {
 	if _, err := os.Stat(dir); errors.Is(err, os.ErrNotExist) {
-		err := os.Mkdir(dir, os.ModePerm)
+		err := os.MkdirAll(dir, os.ModePerm)
 		if err != nil {
 			return fmt.Errorf("failed to create directory %s: %w", dir, err)
 		}