Resolves (hopefully) #328

I've adjusted the reader to be more programmatic and less CSS-centric for computing the navigation tap boundaries:

• If the loaded image is >= 80% of the width of the viewport, the navigation divs will be fixed to the left/right with a fixed 10% width
• If the loaded image is < 80%, the navigation divs will be inline left/right to the image and fill any available space

I've also added swipe handlers on mobile viewport sizes. I tested this on an iPad mini, and then a few other tablet sizes in my dev tools.