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']
    }
  });
```
Use exports instead of availableModules.
Support modules as options with deprecation message.
Include assetPath in deprecation message.
Merge pull request #1121 from abuiles/refactor-import

Allow to pass options to import.