add option for working on multiple tmux server

[sohorx]

Hi,

The behavior described in https://github.com/tmux-plugins/tmux-continuum/blob/master/docs/multiple_tmux_servers.md is something I didn't expect and actually bothering me.

I'm running multiple session of tmux where each have their one configuration files for specific behavior and also a different tmux resurect path.

So the protection of the number_tmux_processes_except_current_server had to be tweaked for echoing 0...

This is kind of ugly and having a simple variable for disabling this protection would be great.