Incorporate / port .skip patch

Original commit: https://github.com/austinbv/impress.js/commit/f6aaa9f7d9700f362796464a0e3dc88a51bff8eb

Original commit message:

  Skippable positions

  It's nice to be able to remain consistent with the markup for background
  stuff but maybe you don't want it in a slide. Now you can add .skip to an
  element and it will be positioned but not scrolled to. Maybe a little hacky
  but it gets the job done right!