add: ctx.locals as a recommended namespace for passing information to the frontend

Just throwing this PR out there. It was discussed in: https://github.com/koajs/koa/issues/338.

Basically this gives you a namespace to attach to that you don't have to worry about this object being undefined. Alternatively, you could always add: 

```js
app.use(function *(next) {
  this.locals = this.locals || {};
  yield next;
});
``` 

at the top of your stack. I feel like this is common enough use-case that it should be added to core, but open to your thoughts.