Fix macOS pbcopy/pbpaste not working in tmux

Some issue with pbcopy/pbpaste not being able to access the clipboard
in tmux. There's a macOS specific fix, with Homebrew install:

- `brew install reattach-to-user-namespace`
- https://git.io/vdBnv

.tmux.conf

@@ -3,6 +3,13 @@ unbind C-b
 set -g prefix C-a
 bind C-a send-prefix
 
+# Set default shell
+set -g default-shell $SHELL
+
+# macOS fix for pbcopy/pbpaste
+if-shell "uname | grep -q Darwin" \
+	'set -g default-command "reattach-to-user-namespace -l ${SHELL}"'
+
 # Better colors
 set -g default-terminal "screen-256color"