Should be able to define more than one validation rule, and rules should allow for an error message

Should be able to do this:

```php
$cmd->option('t')
    ->must(function($t) {
        return is_int($t);
    }, "Option `t` must be an integer")
    ->must(function($t) {
        return $t >= 0 && $t <= 10;
    }, "Option `t` must be between 0 and 10 inclusive.");
```