Use extension if provided in resizer options

[mjauvin]

This is required to allow converting images to other formats (e.g. webp). Otherwise the resulting file will have the correct extension but the wrong image format.

Replaces #9 since the original repo got deleted.
Replaces #74

The following code can be used as a workaround in the meantime:

Event::listen('system.resizer.processResize', function ($resizer, $tempPath) {
    // Get the resizing configuration
    $config = $resizer->getConfig();
    $options = array_get($config, 'options', []);

    list($base, $ext) = explode('.', $tempPath);
    $newPath = $base . '.' . array_get($options, 'extension', $ext);

    // Resize the image
    $resizedImageContents = \Winter\Storm\Database\Attach\Resizer::open($tempPath)
        ->resize($config['width'], $config['height'], $options)
        ->save($newPath);

    if ($newPath != $tempPath) {
        File::move($newPath, $tempPath);
    }

    // Prevent any other resizing replacer logic from running
    return true;
});