add callback option

callback.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"os"
+	"os/exec"
+)
+
+func CallbackUpdateWallpapers(callback string, files, screens []string) (*exec.Cmd, error) {
+	args := []string{}
+
+	for key, file := range files {
+		args = append(args, screens[key], file)
+	}
+
+	cmd := exec.Command(callback, args...)
+	cmd.Env = os.Environ()
+
+	return cmd, cmd.Run()
+}