Allow slides to be uncounted

[lassepe]

Sometimes it is useful to mark slides as uncounted, e.g. if one wishes to add backup/appendix slides but want's the progress bar to reflect the progress of "non-appendix" slides (c.f. #1341). This PR adds the possibility of marking a slide with class .uncounted to achieve this. Currently people seem to use hacks like #1341 (comment) which has other side affects and only works for horizontal slides.

[hakimel]

This has been merged with some changes. Instead of <section class="uncounted"> you'll need to add <section data-visibility="uncounted">. I switched to an attribute because so that we can reuse the same attribute when we eventually make it possible to fully hide a slide with data-visibility="hidden".

Thanks for the PR!