feat(recursion): includes files recursively

## 2dd98e9 closes #1.

By wrapping the HTML processing logic in an infinite loop, we can make the task act recursively.

This means that if file A.html includes B.html, which includes C.html, the final output should contain C.html's content as well.

There is a test for this feature.

## 30eaea1 closes #3.

I modified the behavior of the task to mimick AngularJS's ng-include original behavior. The commit message on 30eaea1 explains this further. 

## Other

30eaea1 also includes a "closing comment": 

```
<!-- ngInclude: ''fixtures/partial-2.html'' -->
<p>bar</p>
<!--/ngInclude: ''fixtures/partial-2.html'' -->
```