Supporting nested sliders

[swisscat]

This pull request introduces a new setting, allowing to handle the following use cases:

<div class="slick">
  <div class="intermediary-element">
    <div class="slick-slide"></div>
    <div class="slick-slide"></div>
    <div class="slick-slide"></div>
  </div>
</div>

By making use of the new option

$('.slick').slick({
    sliderContainer: '.intermediary-element'
});