Add tmux info to shell prompt

prompt_tmux

# .tmux.conf: Disable status bar
set -g status off

# .bashrc: Update prompt info and styling
PS1='\[\033[33m\]$(prompt tmux)\[\033[00m\]'"$format"' \[\033[35m\]$(prompt git)\[\033[00m\]\[\033[31m\]$(prompt job)\[\033[00m\]\n\$ '

tmux)
    # Check for tmux.
    if ! hash tmux 2>/dev/null ; then
        return 1
    fi
    if [[ "$TERM" == screen* ]] && [ -n "$TMUX" ]; then
        printf '[tmux]'
    fi
    ;;