Allows smart replacement for attr blocks such as {dir}, {name}, {ext}

I did a little modification to allow smart modification on [attr] blocks.

I.E.:

```html
  <!-- build:[src] https://ajax.googleapis.com/ajax/libs/angularjs/1.2.7/{name}.min.js-->
  <script src="lib/angular/angular.js"></script>
  <script src="lib/angular/angular-route.js"></script>
  <script src="lib/angular/angular-resource.js"></script>
  <!-- /build -->
```

Will compile to:

```html
  <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.7/angular.min.js"></script>
  <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.7/angular-route.min.js"></script>
  <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.7/angular-resource.min.js"></script>
```

What do you think? For me it's really useful.