add support for {ref} placeholder

ModpackUpdater/ModpackConfig.cs

@@ -13,6 +13,7 @@ public class ModpackConfig
     public string UnleashInstanceId { get; set; }
     public bool PreferDirectLinks { get; set; }
     public string MinecraftVersion { get; set; }
+    public string RefTag { get; set; }
 
     [JsonConverter(typeof(StringEnumConverter))]
     public ModLoader ModLoader { get; set; }