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']
    }
  });
```

The intention behind is to make the `import` helper more extensible, initially this change was going to be part of #919 but it won't be ready between the next couple of days and I see more and more stuff going on with `imports` so we better address this now.