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/Data/Commands/HiddenOptionsCommand.cs

@@ -0,0 +1,12 @@
+using Spectre.Console.Cli;
+
+namespace Spectre.Console.Tests.Data
+{
+    public sealed class HiddenOptionsCommand : Command<HiddenOptionSettings>
+    {
+        public override int Execute(CommandContext context, HiddenOptionSettings settings)
+        {
+            return 0;
+        }
+    }
+}