[Feature Request]: define file name for cover

[roberto-butti]

In order to load jpg file and png file as cover, make asset/cover.jpg configurable.

My proposal is to add image in config['cover']['image']

    /**
     * Cover photo position and dimensions
     */
    'cover' => [
        'position' => 'position: absolute; left:0; right: 0; top: -.2; bottom: 0;',
        'dimensions' => 'width: 210mm; height: 297mm; margin: 0;',
        'image' => 'cover.png',
    ],

Then, replace cover.jpg with the content of ['cover']['image']. Remember to Update the stub/ibis.php file

I'm going to create a PR