Add getUrl and getOpt methods

Add a getOpt function so that it is possible to get the value of a previously set option.
Add a getUrl function so that it is possible to get the last requested URL.

These methods are useful for identifying which request response we are dealing with in each callback method of multiple GET request. `getUrl` can also be used to retrieve the last requested URL of any other Curl instance.

```php
$curl->success(function ($instance) {
    $url = $instance->getUrl();
    // we can know which request response we are dealing with based on URL
});

$curl->get(/* ... */);
```