[tmux] fix lost access to clipboard

First noticed the issue when pasting from the system clipboard stopped
working in vim (only when running inside a tmux session).

The fix is in the following Stack Overflow answer:

    http://stackoverflow.com/a/40154047

The necessary wrapper used by the fixed is at:

    https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard

The wrapper can be installed on the Mac with:

    brew install reattach-to-user-namespace

tmux.conf

@@ -35,6 +35,11 @@ setw -g automatic-rename    on                  # automatically rename windows
 setw -g mouse               on                  # enable mouse support
 setw -g pane-base-index     1                   # start pane numbering from 1
 
+# Fix for clipboard in vim running inside tmux:
+#   See http://stackoverflow.com/a/40154047
+#   See https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard
+set -g default-command "reattach-to-user-namespace -l $SHELL"
+
 #
 # Status bar
 #