Add getOpt function

Adds a getOpt function so that it is possible to get the value of a previously set option.
This is useful for cases like multiple cURL callbacks:

```php
$curl->success(function ($instance) {
    $url = $instance->getOpt(CURLOPT_URL);
    // we can know which request response we are dealing with based on URL
});

$curl->get(/* ... */);
```