Add package_url option

I use Github's releases area for my application releases as here you can upload binaries for download. The url for releases is different to the base_url I use for release_notes and the feed_url.

This patch means that you can use package_url which will give you an enclosure url of:

`<package_url>/<version>/<package_filename>`

Which fits with Github release urls. By not using the option, it will just default back to base_url.