External File support

I created a way to add external file to manifest

```JavaScript
var mergeTrees = require('broccoli-merge-trees');
var manifest = require('broccoli-manifest');

...
  all app.import statements go here
...

// Write a html5 manifest.appcache file with jquery external
var completeTree = app.toTree();
var manifestTree = manifest(completree)
manifestTree.addExternalFile("https://code.jquery.com/jquery-2.1.1.min.js")

module.exports = mergeTrees([completeTree, manifestTree]);
```