Implement `download` command

[Badtz13]

It seems like both #30 and #28 will require mods to be downloaded directly from curseforge. I propose we implement a download command that will be able to download mods for either the client or server.

[maradotwebp]

I don't think a ./pax download command is needed. Sure, at some point we will need the logic for downloading jars from cf, but we don't necessarily need to expose it to the user with a command.