implement simple Move function

mmv.go

@@ -1 +1,13 @@
 package mmv
+
+import "os"
+
+// Move multiple files.
+func Move(files map[string]string) error {
+	for k, v := range files {
+		if err := os.Rename(k, v); err != nil {
+			return err
+		}
+	}
+	return nil
+}