Adds yam & config file

Closes #440 

Adds support for `.ember-cli` settings file which looks like this:

```json
{
  "port": 4200,
  "host": "0.0.0.0",
  "live-reload": true,
  "environment": "development"
}
```

By default, `ember-cli` is going to look for `.ember-cli` file in two places: the current directory and your home directory. You can put your personal settings in your home `.ember-cli` file, team shared settings should stay in project `.ember-cli`.
Looks like the recent changes (to mock yam) caused this to be an unneeded change.

Can you move this back to `tests/helpers/file-utils.js` (with the original contents)?