Add links to original repo when it is available to the plugin manifest

[Hecter94]

For any plugins who still have an active repository, I would like to include a link to that repository in the manifest file, so people know where to look to check for updates, as the archive isn't always updated, and as a way of attributing the original source of the plugin.

Suggested format for link:
[40k Pack](https://github.com/1010todd/Endless-Sky-40K)

[zuckung]

Btw this issue and some others are outdated for a while now.