Add data-fragment-count attribute to allow for simultaneously fragments

When using fragments that have the same index (in order to achieve the look of fragments simultaneously fading in and out) it will cause bugs with the notes view.

Since each data-fragment-index could animate multiple fragments, the fragment index isn't reliable and will cause the notes view to jump ahead fragments.

To combat this I added a `data-fragment-count` attribute. If that attribute is found on a slide, only fragments with that attribute will be counted to get the fragment index.

I thought the effect made for pretty cool presentations, but it made the notes view almost unusable as it wouldn't stay in sync with the actual presentation. I understand that this is an edge case and simultaneous fragment animations might not be worth supporting. I'm also open to other solution to this problem (if it even is worth supporting).

See the example I added to the HTML to see the issue (w/o the fixes in the JS of course).