Allow fallbacks for operating systems and languages

[kneekoo]

I'm exclusively on Linux, so it makes sense to me to only download the Linux version because I have a big enough collection that having all OS versions would take a huge amount of space for something that I might never use.

My default preference goes like:

• if the game is available for Linux, download it
• otherwise download the Windows version

Example syntax: -os linux^windows

The same could be applied to languages: -lang de^en