KAZOO-3116 initial

Callback feature
I would change this to be something more like:

```erlang
CB_Number = ...,
CC_Number = ...,
case whapps_call:request_user(Call) of
  CB_Number -> park_call(JObj, Props, Call);
  CC_Number -> park_call(JObj, Props, Call);
  _RequestUser -> 'ok'
end.

park_call(JObj, Props, Call) ->
    ...
```

Does that make sense? Not that binding variables is expensive, but why build a list of two elements when you can just use pattern matching to do the job, right? As you continue down the Erlang rabbit hole, you will see these patterns more and more, and your code will be clearer for it!