Feature Request: Using grant as a middleware
Looking at the koa consumer: https://github.com/simov/grant/blob/master/lib/consumer/koa.js

It seems this is essentially a wrapper for koa middleware, it would be great if we can use grant as a middleware (attach onto koa `this` context maybe), instead of mounting it as a koa app.

This avoid dependency problem mentioned in #9 and make it a lot more flexible when used with various session backend.
