Fix tmux window name script on Mac
Fix tmux window name script on Mac

It was a case of the mac sed command not having the required
functionality. Fixed by using gsed in the case that it is defined. Would
be nice to find a better pattern for falling back to a different command
if another is not defined.

davidxmoody/dotfiles,davidxmoody/dotfiles,davidxmoody/dotfiles,davidxmoody/dotfiles