Respect absolute path when using html file that are not on root

Hi,

I've updated your lib because I use view layout with expressjs and in this case, index.html is not in public root but in a ``layouts`` folder.

So when I used gulp-usemin to minify my assets
```
public 
+ css/
+ layouts/
   + index.html
```

```
<!-- build:css(dist) /css/app.min.css -->
<link rel="stylesheet" href="/css/app.css">
<link rel="stylesheet" href="/controllers/home/index.css">
<link rel="stylesheet" href="/controllers/404/index.css">
<!-- endbuild -->
```

``app.min.css`` is created in ``layouts/css/``not in ``css/``

My fix concerns only absolute paths, no changes on relative ones.