Add filter allowing change to the default file extension expected

[benvoynick]

This PR adds a hook to change the file format expected by the plugin in its search for images. This can address issue #19 as well as any other format someone would care to use for a site.

[Warface]

Nice work. I've made some change in the main.php so you can put an array of extension instead and load jpg,png and webp.

`
$extensions = ['jpg', 'png', 'webp'];

// Loop through each extension to check if the image exists
foreach ( $extensions as $ext ) {
    $image_path = get_stylesheet_directory() . '/' . $path . '/' . $layout . '.' . $ext;
    $image_uri = get_stylesheet_directory_uri() . '/' . $path . '/' . $layout . '.' . $ext;

    // Check if the image exists for the current extension
    if ( is_file( $image_path ) ) {
        return $image_uri;
    }
}`