Add default value parameter to Confirm extension

Closes #482

test/Spectre.Console.Tests/Unit/AnsiConsoleTests.Prompt.cs

@@ -0,0 +1,29 @@
+using System;
+using Shouldly;
+using Spectre.Console.Testing;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public partial class AnsiConsoleTests
+    {
+        public sealed class Prompt
+        {
+            [Theory]
+            [InlineData(true, true)]
+            [InlineData(false, false)]
+            public void Should_Return_Default_Value_If_Nothing_Is_Entered(bool expected, bool defaultValue)
+            {
+                // Given
+                var console = new TestConsole().EmitAnsiSequences();
+                console.Input.PushKey(ConsoleKey.Enter);
+
+                // When
+                var result = console.Confirm("Want some prompt?", defaultValue);
+
+                // Then
+                result.ShouldBe(expected);
+            }
+        }
+    }
+}