add: battery plugin

plugins/battery/battery.plugin.zsh

+if [[ $(acpi 2&>/dev/null | grep -c '^Battery.*Discharging') -gt 0 ]] ; then
+  function battery_pct_remaining() { echo "$(acpi | cut -f2 -d ',' | tr -cd '[:digit:]')" }
+  function battery_time_remaining() { echo $(acpi | cut -f3 -d ',') }
+  function battery_pct_prompt() {
+    b=$(battery_pct_remaining)
+    if [ $b -gt 50 ] ; then
+      color='green'
+    elif [ $b -gt 20 ] ; then
+      color='yellow'
+    else
+      color='red'
+    fi
+    echo "%{$fg[$color]%}[$(battery_pct_remaining)%%]%{$reset_color%}"
+  }
+else
+  error_msg='no battery'
+  function battery_pct_remaining() { echo $error_msg }
+  function battery_time_remaining() { echo $error_msg }
+  function battery_pct_prompt() { echo '' }
+fi