allows access to the clipboard) on OS X 10.10 and above.
Chis Johnsen has done much work on and documentation of this issue, the code is
copied (with some tweaks) from his reattach-to-user-namespace at:
https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard
Tested by Enrico Ghirardi.
Will see how this goes, if it breaks stuff it might go away again.
