Install add-ons directly from links

Trying to build an add-on that can install other add-ons from links directly.

Currently it installs add-ons from GitHub and links to zip files. Still need to work on handling special cases, since the directory/file structure varies quite a bit across addons.

By the way, I am eagerly looking forward to how the extensions platform will turn out. I believe it would become much simpler to find, install and update add-ons than how we do it now.