[Feature] Support for "external downloads"

[justinrusso]

Since not all mods are on curseforge, would you be able to add support for mods that can be downloaded externally?

I would imagine it being a JSON, txt, or some other file that contains a link to the download location that the modpack developers would create and maintain manually.

[brisingraerowing]

This would be necessary for a pack I'm working on as I include Galacticraft and Computronics in it, and those aren't on Curse.