Added a filter for image helper $image_data

[Liblastic]

This adds a filter for image helper $image_data.

This way we can add attributes for the images added with @image helper.
After the changes this issue will be fixed.
#136

loading attribute can added like so for all the images at once.

/**
 * Filter image helper data.
 *
 * @param array $image_data DustPress image helper data.
 *
 * @return mixed
 */
function filter_image_helper_data( $image_data ) {

    $image_data['attrs']['loading'] = 'lazy';

    return $image_data;
}

\add_action( 'dustpress/image/image_data', 'filter_image_helper_data', 1 );