Allow to pass options to import.

Previously you would do something like

```js
  this.import('vendor/ember-cli-shims/app-shims.js', {
    ember: ['default']
  });
```

Now you will have to pass a key with availableModules:

```js
  this.import('vendor/ember-cli-shims/app-shims.js', {
    availableModules: {
      ember: ['default']
    }
  });
```