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/Settings/HiddenOptionSettings.cs

@@ -0,0 +1,20 @@
+using System.ComponentModel;
+using Spectre.Console.Cli;
+
+namespace Spectre.Console.Tests.Data
+{
+    public sealed class HiddenOptionSettings : CommandSettings
+    {
+        [CommandArgument(0, "<FOO>")]
+        [Description("Dummy argument FOO")]
+        public int Foo { get; set; }
+
+        [CommandOption("--bar", IsHidden = true)]
+        [Description("You should not be able to read this unless you used the 'cli explain' command with the '--hidden' option")]
+        public int Bar { get; set; }
+
+        [CommandOption("--baz")]
+        [Description("Dummy option BAZ")]
+        public int Baz { get; set; }
+    }
+}