Restore ability to use --backend flag

[zackman0010]

Some installations, we only want a web compatible frontend for a single server, so the selection menu allowing it to proxy multiple servers is a bit overkill. The original screeps-steamless-client had the --backend flag, which would automatically redirect to a specific server without having to include it in the URL. I'd like to see the ability to use that flag in screepers-steamless-client as well.