Added the ability to hide CommandOptions. (#642)

CommandOptions now has an IsHidden property that, when set to true, will cause the option to be hidden from the following cases:

- Help text using `-h|--help`
- Xml representations generated with the `cli xml` command
- Diagnostics displayed with the `cli explain` command

Hidden options can still be outputted with `cli explain` using the `--hidden` option that is also used to display hidden commands.

Fixes #631

test/Spectre.Console.Tests/Unit/Cli/CommandAppTests.Xml.cs

@@ -137,6 +137,21 @@ namespace Spectre.Console.Tests.Unit.Cli
                 // Then
                 return Verifier.Verify(result.Output);
             }
+
+            [Fact]
+            [Expectation("Hidden_Command_Options")]
+            public Task Should_Not_Dump_Hidden_Options_On_A_Command()
+            {
+                // Given
+                var fixture = new CommandAppTester();
+                fixture.SetDefaultCommand<HiddenOptionsCommand>();
+
+                // When
+                var result = fixture.Run(Constants.XmlDocCommand);
+
+                // Then
+                return Verifier.Verify(result.Output);
+            }
         }
     }
 }