Mutually exclusive options

It would be great to be able to handle mutually exclusive options.

## Example

For example, if I had a command that either took an `--all` option or a `--tenant ` option, but never both.

```csharp
public static Command Rebuild(IContainer container)
{
var rebuild = new Command("rebuild", "Rebuilds a search index for one (--tenant ) or all (--all) tenants.")
{
Handler = CommandHandler.Create((all, tenant) =>
container.Resolve().ExecuteAsync(all, tenant))
};

rebuild.AddOption(new Option(new[] {"--all", "-a"},
"Indicates if all tenants should be rebuilt.",
new Argument(false) {Arity = ArgumentArity.ZeroOrOne}));

rebuild.AddOption(new Option(new[] {"--tenant", "-t"},
"Specifies the specific tenant that should be rebuilt.",
new Argument {Arity = ArgumentArity.ZeroOrOne}));

return rebuild;
}
```