The 2010s as well as the '69 Cobra or first generation rather had this feature. Many owners agree Ford should have never gotten rid of it, even though Ford has stuck with the trilateral tail lights for much of the make's production.
I highly suggest going to Meter4it Sequentail / Chase turn signals for Ford Mustang
I have them on my car, and the product is top quality, their customer service is top notch (I think it's the same dude that makes them answers all his emails) and he has numerous projects were he has developed the sequential kit for other cars.
It's slightly more money, especially if you pick up the kit with the modified flasher but it is well worth it and it has added features:
* One button flash calibration
And with the switching of a single jumper you can change the light flash setting amoung three (that's right THREE (3) settings!
* There is the sequential, which you have seen
* Chase mode where after they each light up in sequence they turn off in sequence as well
* You can always go back to stock easily with the jumper, don't have to uninstall it.
There are full colored instructions shipped with the kit as well as on his website and the install is quick and easy. He includes everything you need, including the scotch locks for quick splicing (although I personally recommend a better splicing method than these if you can).
I have videos on my old '98 of the chase sequence and I could shoot a quick video on my new edge, which I also have it set to chase.