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/Annotations/CommandOptionAttributeTests.cs

@@ -193,5 +193,25 @@ namespace Spectre.Console.Tests.Unit.Cli.Annotations
             result.ShortNames.ShouldContain("f");
             result.ValueName.ShouldBe("BAR");
         }
+
+        [Fact]
+        public void Is_Not_Hidden_From_Help_By_Default()
+        {
+            // Given, When
+            var result = new CommandOptionAttribute("--foo");
+
+            // Then
+            result.IsHidden.ShouldBeFalse();
+        }
+
+        [Fact]
+        public void Can_Indicate_That_It_Must_Be_Hidden_From_Help_Text()
+        {
+            // Given, When
+            var result = new CommandOptionAttribute("--foo") { IsHidden = true };
+
+            // Then
+            result.IsHidden.ShouldBeTrue();
+        }
     }
 }