Improved debugging

This update is actually to the koa-compose module, an integral component which composes an array of middleware, and is used in core for app.use().

With the use of the debug module you can enable instrumentation of specified modules — in this case when you use DEBUG=koa-compose the request state is written to stdout before and after each middleware. The number on the left indicates the depth, followed by the middleware name, this makes it very easy to determine which middleware has manipulated the response.