Add context propagation middleware

/to @ritch @bajtos @piscisaureus 
This should be IMO `if (options.context !== false)`.

Let me restate my goal:
 - As a developer upgrading an existing loopback application scaffolded via `slc loopback`, I want the context middleware to be automatically enabled after upgrade, so that I can start using all the goodness it brings in other modules
 - As a developer upgrading an existing loopback app written manually, I want the context middleware to be automatically enabled after upgrade. Here is my app:

  ```js
  var app = loopback();
  app.use(loopback.rest());
  app.listen()
  ```

I hope that we have already learned our lesson here when we have added `loopback.token` middleware and people were confused about how to upgrade their existing apps.