Some more CLI options for launching

I'm currently implementing support for AMIDST in MultiMC, similar to the existing MCEdit support. For that I'm requiring some more control over AMIDST than available, as the directory structure of MultiMC is slightly different than that of vanilla Minecraft.

This PR works together with MultiMC/MultiMC5@77251d1378fbfe99a56102bc86ecc3e94551a8c1