Add toggle for whether vanished players are removed from server player count

[Starmism]

As the title states, using ajQueue on a Velocity network, I need the player count to be accurate so that it knows when to stop sending players. I have accomplished this by manually deleting that code from my fork, but that means I have to keep up with upstream manually (which is a rare occasion I actually do!). It would be really great if there were just a config setting to toggle this behavior. I can make a PR for it if nobody else wants to, but my familiarity with the EssentialsX config setup is low and I don't have much time these days. Just wanted to put this issue here as an easy first issue for someone and a welcome improvement to EssentialsX!