Add nomod mod type

osu.Game/Rulesets/Mods/NoModMod.cs

@@ -0,0 +1,15 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+namespace osu.Game.Rulesets.Mods
+{
+    /// <summary>
+    /// Indicates a type of mod that doesn't do anything.
+    /// </summary>
+    public sealed class NoModMod : Mod
+    {
+        public override string Name => "No Mod";
+        public override string ShortenedName => "NM";
+        public override double ScoreMultiplier => 1;
+    }
+}