Target multiple Ember apps from `.ember` config file. 

Basically I have a rails app with multiple Ember applications that I use ember-tools with. I wanted to be able to declare multiple apps in the `.ember` and target them specifically with `ember build specific-app`. 

Here is an example of a new .ember file:
````JSON
{
  "test-app": {
    "modules": "cjs",
    "appDir": "test-app"
  },
  "another-test-app": {
    "modules": "cjs",
    "appDir": "another-test-app"
  }
}
````
Running `ember build` will run the first app in the file, or you can target by using the App name. I incorporate this into my Grunt.js workflow and find it helpful.

Test added that passes.