Extract turn.js out of the starter repo

[pascallaliberte]

See bullet-train-co/bullet_train#221

[domchristie]

FWIW I've released Turn on npm. (You'll still need to include the CSS.)

It also includes a basic way disabled transitions by adding data-turn="false" to the body. This might be handy when transitioning from a feature that uses transitions to one that doesn't. For example, one thing I've noticed about the Bullet Train flow is that the Dashboard doesn't transition after sign in. We could add data-turn-enter to the page with a simple fade-in animation, and also add data-turn="false" to disable transitions from that screen.

What do you reckon?