Correct exit from tmux display-message

[Boruch-Baum]

1. In your file 'tmux_echo_functions.sh', you incorrectly direct the user to press ENTER to continue. It should be 'q'.

2. Also, you're having tmux spawn a bash process to echo a message to the screen when tmux has its own internal command 'display message -p'.

3. For me, this is too minor to clone and make a pull request. The code is:

tmux_echo() {
    local message="$1"
    tmux display-message -p "$message'"
}

echo_ok() {
    tmux_echo "$*"
}

echo_err() {
    tmux_echo "$*"
}

end_message() {
    tmux_echo "
TMUX environment reloaded.

Done, press 'q' to continue."
}

4. Thanks for a great addition to the already great tmux