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