Feature: support for Koa 2.0

[HowlingEverett]

Koa 2.0 is moving away from generator functions completely, which came as a bit of a surprise. The ultimate end-goal appears to be building the middleware around ES7 async/await. That works in the alpha.2 already when you run with babel, but in the interim it seems like tj is expecting package authors to switch from generator middleware functions to Promises.

A lot of middleware can be hacked into compatibility simply by calling co.wrap around the initialiser, but koa-handlebars is a bit more complex since it's augmenting the context with functions that other route handlers use.

I'm happy to help out with this once I get my head around the new API, since koa-handlebars is the only piece of middleware in my current project that isn't working with the koa 2.0 alpha yet.

[chrisveness]

With Node.js v7 arriving later this month, which has async/await implemented behind a --harmony flag, Koa v2 will be usable without Babel.

It would be great to see koa-handlebars have Koa v2 support (presumably published @next until async/await emerges from behind its flag).

[dominicbarnes]

I'd gladly accept a PR here, as I don't really maintain this much these days.