adding an envfile or similar to change default behavior

[tcsenpai]

I think it would be nice to have an .env file or similar (or something to pass to umu-run by the way) that allows us to choose for example the default sniper download path and other choices. This would be useful for launchers and 3rd party tools too, providing a degree of customization that may be useful.

Right now umu_consts.py contains many of the default options as constants (indeed). With a more flexible approach, launchers can ship their runtimes too or have multiple runtimes if they need to.