allow adding a prefix to generated paths

I've been using usemin with Jekyll and github pages and in order to be able to develop and run the site locally (without running usemin, so as to debug) and also push the generated site (after running usemin) I needed a way to add a prefix (```baseurl```, from Jekyll) to my assets' paths.

This patch allows the user to define such prefix in the usemin options and then makes sure it gets added to all affected files. 

This option..
```
usemin: {
    options: {
        prefix: '/project-name'
    }
}
```

and this markup..

```
<!-- build:js /scripts/scripts.min.js -->
<script type="text/javascript" src="/scripts/jquery.js"></script>
<script type="text/javascript" src="/scripts/bootstrap.js"></script>
<!-- endbuild -->
```

result to this:

```
<script src="/project-name/scripts/f280ae41.scripts.min.js"></script>
```