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/system-info-macOS.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+cache_dir=~/.cache/tmux
+cache_file=$cache_dir/system-info
+
+# Make sure the output directory exists.
+if [ ! -d $cache_dir ]; then
+  mkdir -p $cache_dir
+fi
+
+# Cleanup cache file when interrupted.
+trap '[ -f $cache_file ] && rm $cache_file; exit' INT
+trap '[ -f $cache_file ] && rm $cache_file; exit' TERM
+
+while true; do
+  battery="`pmset -g batt | grep --color=never -Eo '\d+%'` ↯"
+  echo "$battery" > $cache_file
+  sleep 2
+done