Introducing `config` hook for addons

Implements: #1907

A new `config` hook is available for addons. It can be used to alternate application configuration (example: toggling FEATURE flags for Ember) or to read the current config and change addon behaviour (example: importing dependencies conditionally).

Interface:
```
// environment - 'development', 'test', 'production'
// currentConfig - hash with config for given environment
config: function(environment, currentConfig) {}
```
it is pretty tricky to make `config` hook fire before `included` hook is. Not sure when exactly `ConfigLoader#updateCache` is being called.