light clock (#260)

* extend image effects
add a basic light clock

* add alpha values  to rainbow

* some effect tuning

* make seconds hand optional and change color to yellow
make blue hours hand always visible

effects/light-clock.json

@@ -0,0 +1,8 @@
+{
+	"name" : "Light clock",
+	"script" : "light-clock.py",
+	"args" :
+	{
+		"show_seconds" : true
+	}
+}