Add option to center target slide

[dairiki]

Add boolean option centerSlide. When set, the active slide is centered within the slider, rather than positioned on the left side of the slider.

This is a re-implementation of #144.

[humbertorodrigues]

Hello!

Any idea how to center even when it's the last/first image?

[haferburg]

I just tried it out, thank you for the PR. Unfortunately it doesn't work properly. I use {loop: true, autoWidth: true, centerSlide: true}. Which options did you use when you tested?

What does work: On page load, the active slide is centered. The arrow navigation buttons and the dots below do work properly, so that's great.

What doesn't work: When I drag/swipe, it still takes the left-most item and activates it. The active item is then centered, resulting in a big jump, and jerky behavior.

As a side note, I think centerActive would be a better name for this option.

[IlyaBuldakov]

up