Add partial view of prev and next slide

[arcxyz]

For a lot of mobile projects I have a hardcoded version of swipe to make this. I've decided to fork your repo and add the "partial" option. Check it out:

Demo: http://arcxyz.github.io/swipe/
Code: https://github.com/arcxyz/swipe

It only works with 3+ slides and probably only with continuous option.

[lyfeyaj]

@arcxyz Nice Feature!
I create a PR from your repo, and will review it later. But there is a little bug: lets say there are 3 slides, starting at 1, when click next, 1 is moving faster than 2.

[arcxyz]

I made some changes to the slide function, could you check it now?

[morer]

@arcxyz Great Job
But I found that if I change the ratio of each slide, like this:
width = options.partial ? containerWidth*0.8 : containerWidth; element.style.width = (slides.length * width) + 'px'; partialPos = { hidden: containerWidth, prev: -containerWidth*0.7, middle: containerWidth*0.1, next: containerWidth*0.9 };

I clicked next and the next slide does not appear, and then I checked slide direction definition

var direction = Math.abs(index-to) / (index-to); // 1: backward, -1: forward

and I change
if (options.partial) { if (direction === -1) {
to

if (options.partial) { if (natural_direction === -1) {
in function slide, the next slide shows up.