add slow rainbow effect

ledstring.yml

@@ -72,6 +72,10 @@ light:
           name: rainbow
           speed: 10
           width: 50
+      - addressable_rainbow:
+          name: slowrainbow
+          speed: 1
+          width: 70
       - addressable_flicker:
           name: flicker
           update_interval: 50ms