Add attribute to be able to pass scope for the custom paging

[mamzellejuu]

I added an attribute for the directive to be able to pass scope the custom paging.

<slick infinite="true" autoplay="false" centerMode="true" dots="true", prev-arrow=".slick-prev" next-arrow=".slick-next" custom-paging="customPaging(slick, index)" custom-paging-scope="useScope"></slick>

$scope.customPaging = (slick, index) => {
      let $currentSlide = $(slick.$slides[index]);
      let url = $currentSlide.data('analytics-event');
      let label = $currentSlide.data('analytics-label');
      let category = $currentSlide.data('analytics-category');

      return '<button type="button" ng-click="ngFn()" data-role="none" role="button" tabindex="0"  analytics-on="click" analytics-event="' + url + '" analytics-category="' + category + '" analytics-label="' + label + '">' + index + '</button>';
    }

It may be useful in some cases to have access to a different scope as in the example.