add checkboxes filter to admin filters

[kokiddp]

I added the possibility to setup multiple admin taxonomy filters, implemented with a fieldset of checkboxes
https://github.com/johnbillion/extended-cpts/wiki/Admin-filters#taxonomy-terms-dropdown

'admin_filters' => [
    'genre' => [
        'title'    => 'Genre',
        'taxonomy' => 'genre',
        'type'     => 'checkboxes'
    ],
],

[johnbillion]

Thanks for the PR @kokiddp! I won't have time to check this for a while, but I've added it to my todo list :-)