Add protonup

This MR adds protonup, a little Python utility to help install and manage GloriousEggroll's Proton version (probably the most popular community Proton version). This does not need to be run in the Steam container and can work for any Steam version, just by setting the right directory for protonup to use.

The only slight wrinkle is that the maintainer hasn't been active for several months, so the source is using a pending pull request. This can be changed back if/when the original author reappears, or else the homepage can be set to this fork if it goes that way.