Seal "LeafPattern" subclasses

Author: atifaziz

src/DocoptNet/Internals/Argument.cs

@@ -5,7 +5,7 @@ namespace DocoptNet.Internals
 {
     using System.Globalization;
 
-    class Argument: LeafPattern
+    sealed class Argument : LeafPattern
     {
         public Argument(string name) : base(name, ArgValue.None) { }
 

src/DocoptNet/Internals/Command.cs

@@ -3,7 +3,7 @@
 
 namespace DocoptNet.Internals
 {
-    class Command : LeafPattern
+    sealed class Command : LeafPattern
     {
         public Command(string name, bool value = false) :
             base(name, value ? ArgValue.True : ArgValue.False) { }

src/DocoptNet/Internals/Option.cs

@@ -6,7 +6,7 @@ namespace DocoptNet.Internals
     using System;
     using System.Text.RegularExpressions;
 
-    partial class Option : LeafPattern
+    sealed partial class Option : LeafPattern
     {
         public string ShortName { get; private set; }
         public string LongName { get; private set; }