Added variable to tmux's update-environment list
Created TTY-aware display function for metamux

- Added "display" function to metamux that prints messages to stderr
  when using metamux in a terminal and uses tmux's "display" command
  otherwise.
Added environment cloning function to metamux

- Metamux now has an "env_clone" function that can be used to
  reinitialized tmux's environment variable by siphoning them from an
  existing process.