Add navigation buttons functionality

[hyperstown]

Hi I've added option to easy add navigation buttons to the slider.
Now user can specify in options navButtons: true to easy add previous and next button. Button text can also be customized using navNextText and navPrevText parameters in options.

var mySwipe = new Swipe(document.getElementById('slider'), {
  startSlide: 0,
  speed: 400,
  draggable: true,
  continuous: false,
  navButtons: true, // default false
  navNextText: 'next', // default value
  navPrevText: 'prev', // default value
  ignore: ".scroller",
});