tmux: implement ENVIRONMENT

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
tmux: implement Status Line

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
tmux: use status-left short flags

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>

Conflicts:
	share/completions/tmux.fish
tmux: make session auto-completion user friendly

make it more user friendly by matching the output of:
  tmux list-sessions

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>