support for koa2

[XueSeason]

I rewrite some code for support koa2, but no time to write test code.

[shellscape]

Looks like you didn't lint your code before submitting the PR. Travis is failing with linting errors.

When submitting a PR to a project, you don't want to change the style of the code to your own preferences. It looks like your editor removed semicolons.

[kevinrambaud]

hey @XueSeason your rewrite has already been done and is available on the "next" branch (https://github.com/koajs/koa-hbs/tree/next)

you can install the next version by running

npm install koa-hbs@next --save

[shellscape]

I was gonna lead them to that, but they never responded 😄