Feature: Support multiple galleries

I ran into a situation where I wanted multiple separate galleries on a page. This can be accomplished using the sample code provided on the Magnific Popup website https://dimsemenov.com/plugins/magnific-popup/documentation.html#gallery

I edited the JS for galleries in wpmp.js in the following way.

jQuery('.' + wpmp_options.gallery.class).each(function() {
    jQuery(this).magnificPopup({

        ...

    });
});