Resolve triple-slashes in processed GitHub URLs

Parsing would yield invalid URLs in https:// and git:// casees, e.g.

    https://github.com/sethvargo/stove.git =>
        https///github.com/sethvargo/stove

    git://github.com/sethvargo/stove.git =>
        git///github.com/sethvargo/stove
Make community site interaction into a plugin

This makes it possible to release to any combination of upload
destinations, e.g. for someone who's ready to tag a release in GitHub,
but doesn't necessarily want to do a community site upload.