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) {}
```
Refactor addon config generation to Project.prototype.config.
Merge pull request #1972 from rwjblue/addon-config

Addon configuration.
Add default `Addon.prototype.config`.
Remove unused root level fixtures folder.
Merge pull request #1978 from rwjblue/remove-extraneous-fixture-file

Remove unused root level fixtures folder.
Merge pull request #1977 from rwjblue/add-default-config-implementation

Add default `Addon.prototype.config`.
Removed broccoli-sweetjs from registry since it doesn't work.

If you try and use it, you'll simply get `Unknown statement type: ImportDeclaration` because sweetjs doesn't know about ES6 modules yet.