🐛 Exclude cover image from double spread and fix rtl

[smldub]

• The first page is the cover, so it should be shown by itself instead of in double spread (maybe this could be optional)
  • requires skip variable to make it possible to go back to page 1 (else it will try to subtract two pages from page 2)
• Reading right to left vs left to right requires flipping the ordering of the pages

(never coded a line of javascript before tonight, so sorry if it the code is awful...)

[aaronleopold]

Thanks so much for fixing this! I'm traveling for work but will give it a proper look once I return