Add demo project

CliFx.Demo/Models/Extensions.cs

@@ -0,0 +1,22 @@
+using System.Linq;
+
+namespace CliFx.Demo.Models
+{
+    public static class Extensions
+    {
+        public static Library WithBook(this Library library, Book book)
+        {
+            var books = library.Books.ToList();
+            books.Add(book);
+
+            return new Library(books);
+        }
+
+        public static Library WithoutBook(this Library library, Book book)
+        {
+            var books = library.Books.Where(b => b != book).ToArray();
+
+            return new Library(books);
+        }
+    }
+}