Allow list of custom download links for redudancy

[gcask]

Being able to specify multiple custom download links would allow repository owners to maintain alternative download sites/backups in case one of the links go down.

It would also enable the community to gradually share repository: in the context of DCS for instance, a squadron could use a mod's primary download site as the first custom URL, and maintain backups as alternative URLs in case the primary one goes dead.

Pushing further, it would enable options for repository owners to load balance across multiple backend storage (GDrive / Dropbox / own host, etc) if the repository also supported a 'randomize download URL' in combination with the above.

[lmiol]

I think it is useful, but not first priority. I mean 1 link is still works fine, because people can download. Other good features is more needful. But final decision by app author of course :D