Add system-info macOS user agent

The `system-info` directory contains scripts and configuration files to
install and remove a `launchd` user agent on macOS which collects
information about the system and outputs it to the file
`~/.cache/tmux/system-info`. This file is then read periodically by
`tmux` and the contents displayed in the status line.

system-info/remove.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+if [ `uname` = Darwin ]; then
+  launchctl unload ~/Library/LaunchAgents/system-info.plist
+  rm ~/Library/LaunchAgents/system-info.plist
+fi