Allow using environment variables in command.

Specifically, I would like to be able to do things like this:

``` js
    {
      name: 'ember-alpha',
      command: 'SPECIAL_SAUCE=true ember test',
      bower: {
        dependencies: {
          'ember': 'alpha'
        },
        resolutions: {
          'ember': 'alpha'
        }
      }
    }
```

or:

``` bash
ember try:one ember-alpha --- SPECIAL_SAUCE=true ember test
```
