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/Expectations/Cli/Help/Hidden_Command_Options.Output.verified.txt

@@ -0,0 +1,9 @@
+USAGE:
+    myapp <FOO> [OPTIONS]
+
+ARGUMENTS:
+    <FOO>    Dummy argument FOO
+
+OPTIONS:
+    -h, --help    Prints help information
+        --baz     Dummy option BAZ

test/Spectre.Console.Tests/Expectations/Cli/Xml/Hidden_Command_Options.Output.verified.txt

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Model>
+  <!--DEFAULT COMMAND-->
+  <Command Name="__default_command" IsBranch="false" IsDefault="true" ClrType="Spectre.Console.Tests.Data.HiddenOptionsCommand" Settings="Spectre.Console.Tests.Data.HiddenOptionSettings">
+    <Parameters>
+      <Argument Name="FOO" Position="0" Required="true" Kind="scalar" ClrType="System.Int32">
+        <Description>Dummy argument FOO</Description>
+      </Argument>
+      <Option Short="" Long="baz" Value="NULL" Required="false" Kind="scalar" ClrType="System.Int32">
+        <Description>Dummy option BAZ</Description>
+      </Option>
+    </Parameters>
+  </Command>
+</Model>