Adds a method that print a battery gauge

It is especially useful for themes, this function is a port of the code written by Steve Losh
and made available on [this blog entry](http://stevelosh.com/blog/2010/02/my-extravagant-zsh-prompt/#my-right-prompt-battery-capacity)

plugins/battery/battery.plugin.zsh

@@ -11,8 +11,9 @@
 if [[ $(uname) == "Darwin" ]] ; then
 
   function battery_pct() {
-    typeset -F maxcapacity=$(ioreg -rc "AppleSmartBattery"| grep '^.*"MaxCapacity"\ =\ ' | sed -e 's/^.*"MaxCapacity"\ =\ //')
-    typeset -F currentcapacity=$(ioreg -rc "AppleSmartBattery"| grep '^.*"CurrentCapacity"\ =\ ' | sed -e 's/^.*CurrentCapacity"\ =\ //')
+    local smart_battery_status="$(ioreg -rc "AppleSmartBattery")"
+    typeset -F maxcapacity=$(echo $smart_battery_status | grep '^.*"MaxCapacity"\ =\ ' | sed -e 's/^.*"MaxCapacity"\ =\ //')
+    typeset -F currentcapacity=$(echo $smart_battery_status | grep '^.*"CurrentCapacity"\ =\ ' | sed -e 's/^.*CurrentCapacity"\ =\ //')
     integer i=$(((currentcapacity/maxcapacity) * 100))
     echo $i
   }
@@ -26,8 +27,9 @@ if [[ $(uname) == "Darwin" ]] ; then
   }
 
   function battery_time_remaining() {
-    if [[ $(ioreg -rc AppleSmartBattery | grep -c '^.*"ExternalConnected"\ =\ No') -eq 1 ]] ; then
-      timeremaining=$(ioreg -rc "AppleSmartBattery"| grep '^.*"AvgTimeToEmpty"\ =\ ' | sed -e 's/^.*"AvgTimeToEmpty"\ =\ //')
+  	local smart_battery_status="$(ioreg -rc "AppleSmartBattery")"
+    if [[ $(echo $smart_battery_status | grep -c '^.*"ExternalConnected"\ =\ No') -eq 1 ]] ; then
+      timeremaining=$(echo $smart_battery_status | grep '^.*"AvgTimeToEmpty"\ =\ ' | sed -e 's/^.*"AvgTimeToEmpty"\ =\ //')
       echo "~$((timeremaining / 60)):$((timeremaining % 60))"
     else
       echo "∞"
@@ -49,6 +51,10 @@ if [[ $(uname) == "Darwin" ]] ; then
       echo "∞"
     fi
   }
+  
+  function battery_is_charging() {
+	  [[ $(ioreg -rc "AppleSmartBattery"| grep '^.*"IsCharging"\ =\ ' | sed -e 's/^.*"IsCharging"\ =\ //') == "Yes" ]]
+  }
 
 elif [[ $(uname) == "Linux"  ]] ; then
 
@@ -79,14 +85,66 @@ elif [[ $(uname) == "Linux"  ]] ; then
       echo "∞"
     fi
   }
+  
+  function battery_pct() {
+    # todo for on linux
+  }
+  
+  function battery_is_charging() {
+    # todo on linux
+	false
+  }
+
 else
-	# Empty functions so we don't cause errors in prompts
-	function battery_pct_remaining() {
-	}
+  # Empty functions so we don't cause errors in prompts
+  function battery_pct_remaining() {
+  }
 
-	function battery_time_remaining() {
-	}
+  function battery_time_remaining() {
+  }
 
-	function battery_pct_prompt() {
-	}
+  function battery_pct_prompt() {
+  }
 fi
+
+function battery_level_gauge() {
+  local gauge_slots=${BATTERY_GAUGE_SLOTS:-10};
+  local green_threshold=${BATTERY_GREEN_THRESHOLD:-6};
+  local yellow_threshold=${BATTERY_YELLOW_THRESHOLD:-4};
+  local color_green=${BATTERY_COLOR_GREEN:-%F{green}};
+  local color_yellow=${BATTERY_COLOR_YELLOW:-%F{yellow}};
+  local color_red=${BATTERY_COLOR_RED:-%F{red}};
+  local color_reset=${BATTERY_COLOR_RESET:-%{%f%k%b%}};
+  local battery_prefix=${BATTERY_GAUGE_PREFIX:-'['};
+  local battery_suffix=${BATTERY_GAUGE_SUFFIX:-']'};
+  local filled_symbol=${BATTERY_GAUGE_FILLED_SYMBOL:-'▶'};
+  local empty_symbol=${BATTERY_GAUGE_EMPTY_SYMBOL:-'▷'};
+  local charging_color=${BATTERY_CHARGING_COLOR:-$color_yellow};
+  local charging_symbol=${BATTERY_CHARGING_SYMBOL:-'⚡'};
+
+  local battery_remaining_percentage=$(battery_pct);
+
+  if [[ $battery_remaining_percentage =~ [0-9]+ ]]; then
+    local filled=$(((( $battery_remaining_percentage + $gauge_slots - 1) / $gauge_slots)));
+    local empty=$(($gauge_slots - $filled));
+	
+    if [[ $filled -gt $green_threshold ]]; then local gauge_color=$color_green;
+    elif [[ $filled -gt $yellow_threshold ]]; then local gauge_color=$color_yellow;
+    else local gauge_color=$color_red;
+    fi
+  else
+    local filled=$gauge_slots;
+    local empty=0;
+  	filled_symbol=${BATTERY_UNKNOWN_SYMBOL:-'.'};
+  fi
+
+  
+  local charging=' ' && battery_is_charging && charging=$charging_symbol;
+
+  printf ${charging_color//\%/\%\%}$charging${color_reset//\%/\%\%}${battery_prefix//\%/\%\%}${gauge_color//\%/\%\%}
+  printf ${filled_symbol//\%/\%\%}'%.0s' {1..$filled}
+  [[ $filled -lt $gauge_slots ]] && printf ${empty_symbol//\%/\%\%}'%.0s' {1..$empty}
+  printf ${color_reset//\%/\%\%}${battery_suffix//\%/\%\%}${color_reset//\%/\%\%}
+}
+
+