Add option to use a single wrapper to apply scale transform to slides

[mojavelinux]

When the "transform" option is specified, bespoke-scale wraps an element around each slide individually to apply the scale transform. I don't think this is necessary (and may be impacting performance). I think a single element can be used that wraps all the slides, thus applying the scaling using a single transform.

For example:

<div class="bespoke-scale-parent" style="transform: scale(1.5);">
  <section class="bespoke-slide bespoke-active">
    <h1>Slide 1</h1>
  </section>
  <section class="bespoke-slide bespoke-inactive bespoke-after bespoke-after-1">
    <h1>Slide 2</h1>
  </section>
  ...
</div>

If we want to keep both options of wrapping either the group of slides or each slide individually, I propose that we introduce the option value "transform-group" which will add this new behavior. The existing values will remain unchanged.