Add gulp header plugin

[zertosh]

@zpao, I was (fully) porting React from grunt to gulp, and I figured that this makes sense as a gulp plugin and that it should live here.

cc: @cpojer

[zertosh]

I gave it the full node-4.0 compatible ES6 treatment - no destructuring. And since we can count on modern streams and use class, I ditched through2. If this is good, then I'll squash my changes.