twitter and another third party libraries include async

i had this issue with the twitter script that takes so long to load, and results in a very slow page loading

```
<script src="http://platform.twitter.com/widgets.js"></script>
```

i tried to add this script async but i got errors from the ng-embed m because twitter is not found , it  would be good if ng-embed takes care of adding the twitter lib when twitter : true , and do it async
