Make Extensions manager support zipped extensions and multiple repositories (local, github)

This is a draft version, in order to discuss about it.
Some improvments are still needed (allow to add a extension version for example), but it already gives a good idea.
The goal is to be able to install extensions stored on others repositories, like github.
To work properly, conventions have to be defined on the location of the extensions. For example, for the Github exo-addons repository, I used the conventions that the zipped extension is attached (an asset) to a release/tag. I did it for the extension video-wiki-macro, so I can install it  by a simple "./extension.sh --install video-wiki-macro".

WDYT ? Is it useful ?