Drop the use of nohup on OSX for tmux compatibility

Apparently `nohup` is not needed in OSX, and using it has the side
that it may break under tmux with the error

  nohup: can't detach from console: No such file or directory

For now, this commit only drops the use of `nohup` in OSX. But it
*may* not be necessary under other similar platforms.

lib/functions.zsh

@@ -28,7 +28,10 @@ function open_command() {
               ;;
   esac
 
-  nohup $open_cmd "$@" &>/dev/null
+  # don't use nohup on OSX
+  [[ "$OSTYPE" != darwin* ]] && open_cmd="nohup $open_cmd"
+
+  $open_cmd "$@" &>/dev/null
 }
 
 #
@@ -220,4 +223,3 @@ function omz_urldecode {
 
   echo -E "$decoded"
 }
-