Add a --download-dir arguement

[thejudge156]

Doesn't download assets or anything, but downloads libraries and the main jar files for a selected version.

I would like this as part of my MCNative exploration, which is GraalVM Native Image with Minecraft.

[mindstorm38]

This is highly specific! I don't really like this idea of download directory. Instead, I prefer to improve and generalize the installation directories, for example we currently only have --dry to dry run the installer, we could have a more precise way to specify which parts to download (assets, libs, ...). To sum up this may be too specific, we need more time to think of a correct way to do that, because the intent of this launcher is to be dir-layout compatible with official launcher (except for JVMs).