PluginDownloader: support for more hosting services

[jlu5]

It would be nice to support sources that aren't exclusively GitHub:

• [ ] GitLab
• [ ] Gogs / Gitea
• [ ] Launchpad

As far as I can tell, all of these services offer tarball downloads, so we should probably abstract out the extracting part of the code anyways.

edit: or we can scrap vendor-specificness entirely and start using shallow Git clones