Enhancement: Use gestures to navigate between comics

[tcoxon]

One user emailed and suggested adding gestures to navigate between comics: swipe left / right to go to next / previous; down to go to a random comic, etc.

Could potentially interfere with scrolling, but the user pointed out that feedly, for example, apparently supports both scrolling and gestures. Gestures are performed with quick short movements and scrolling with slower long movements.