Add '--prefix' switch

This allows writing files to an arbitrary path.

Perhaps related to #9, one use-case is support for Platform
(http://getplatform.com) extensions. Platform extensions are vanilla Laravel
bundles, namespaced from the project root like so:

```
$ tree
.
|-- application
|-- bundles
|-- extensions
|   |-- [company]
|   |   `-- [extension]
|   |       |-- controllers
|   |       |-- migrations
|   `...    `...
`...
```

With this patch, you can do something like:

    $ php artisan bob::build c test --prefix=company/extension