[Suggestion] Add Tmux install to ProxmoxVE post install script

[slightlyevolved]

As the title says. No need to make it default or anything, but having tmux installed on the PVE is useful, as if you are going to be running along update, etc, being able launch tmux first in case the web shell connection drops or something then being able to reattach to the session could be a lifesaver.

I already do this to every PVE server I spin up.