Feature: add user filter option to filter out any user's temporary/background sessions

[poetaman]

Add a way of specifying a user filter for tmux list-sessions command in the following function of https://github.com/tmux-plugins/tmux-sessionist/blob/master/scripts/kill_session.sh.

Advantage: User can filter out any temporary sessions they might create during the run. For instance, I create a scratch session to hold a scratch pane, and would not want that to be accounted for while either while switching or deciding to not quit tmux. -f "#{!=:#{session_name},scratch}"

For instance,

number_of_sessions() {
	tmux list-sessions |
		wc -l |
		sed "s/ //g"
}

would become:

number_of_sessions() {
	tmux list-sessions -f "#{!=:#{session_name},scratch}" |
		wc -l |
		sed "s/ //g"
}