add reftag commandline argument

ModpackUpdater.Apps.Client/Options.cs

@@ -11,6 +11,7 @@ internal class Options
     public bool Help { get; private set; }
     public bool Silent { get; private set; }
     public bool NoUi { get; private set; }
+    public string RefTag { get; private set; }
     public UpdateCheckOptionsAdv UpdateOptions { get; } = new();
 
     public Options(string[] args)
@@ -28,6 +29,7 @@ internal class Options
             { "m|maintenance", "Ignores the maintenance mode.", m => UpdateOptions.IgnoreMaintenance = true},
             { "i|nonpublic", "Include non public (currently hidden) updates.", i => UpdateOptions.IncludeNonPublic = true},
             { "k|key=", "An key for retriving extra files on updates.", k => UpdateOptions.ExtrasKey = k},
+            { "r|reftag=", "Force uses a specific version, if supported.", r => RefTag = r},
         };
 
         additionals.AddRange(options.Parse(args));