Allow multiple instances to be synced

[jakiestfu]

Unfortunately, not all tests are passing. Two are failing, and I'm scratching my head a little bit as to why...

[desandro]

Thank you for this contribution! This could be a nice little feature 😄

Rather than modifying the source, did you try using sync method instead? See demo on CodePen

var flktyA = new Flickity('.carousel-a');
var flktyB = new Flickity('.carousel-b');
var flktyC = new Flickity('.carousel-c');

flktyA.sync('.carousel-b');
flktyA.sync('.carousel-c');

I'll keep this PR open for a bit in case others are interested.

[jakiestfu]

Interesting point. My team and I will futz around with this and see if it solves our use case. This would obviously be a simpler and more practical solution. Will post updates soon!