Add `get()` and `context` to debugger helper

* `get` gets a value from a stream based on a path
* The context variable lets users know how to get the context object of
  the template

Related to https://github.com/emberjs/ember.js/issues/9960

I went into this thinking I could find a way to unit test this and quickly found out why this helper was untested. Let me know what you think.