Add a setting for critical battery level and default it to 5%

Some batteries do not set their levels, so we need a way to tell the widget to
display a "low battery" warning.

README.md

@@ -68,6 +68,9 @@ This can be done by changing the `gui_client` field of the widget. The default
 is to have no client. For example, you could use the [XFCE4 Power Manager](http://goodies.xfce.org/projects/applications/xfce4-power-manager)
 or the [GNOME one](https://projects.gnome.org/gnome-power-manager/).
 
+You can set the critical battery percentage at which a warning will be
+displayed using the `critical_percentage` property (defaults to `5`).
+
 # Mouse controls
 
 When the widget is focused:
@@ -88,8 +91,10 @@ Require the module:
 -- require *after* `beautiful.init` or the theme will be inconsistent!
 local power = require("power_widget")
 -- override the GUI client.
-power:init()
 power.gui_client = "xfce4-power-manager"
+-- override the critical battery percentage
+power.critical_percentage = 18
+power:init()
 ```
 
 Add the widget to your layout:

power_widget.lua

@@ -33,6 +33,7 @@ icon_theme_dirs = beautiful.upower_icon_theme_dirs or icon_theme_dirs
 icon_theme_extensions = beautiful.upower_icon_theme_extension or icon_theme_extensions
 
 local widget = wibox.widget.imagebox()
+widget.critical_percentage = 5
 
 local function build_icon_path(device)
   if device.IconName then
@@ -54,10 +55,15 @@ function widget:update()
     self.tooltip:set_text(
       percentage .. "%" .. " - " .. self.device.state.name)
 
-    if warning_level == WarningLevel.Low or warning_level == WarningLevel.Critical then
+    if (
+        percentage <= self.critical_percentage
+        or warning_level == WarningLevel.Low
+        or warning_level == WarningLevel.Critical
+    ) then
+      local msg = (warning_level.name == "None" and "Low" or warning_level.name) .. " battery!"
       naughty.notify({
           preset = naughty.config.presets.critical,
-          title = warning_level.name .. "  battery!",
+          title = msg,
           text = percentage .. "% remaining"})
     end
   else

rockspec/power_widget-0.3.4-1.rockspec

@@ -0,0 +1,27 @@
+package = "power_widget"
+version = "0.3.4-1"
+source = {
+   url = "git://github.com/stefano-m/awesome-power_widget",
+   tag = "v0.3.4"
+}
+description = {
+   summary = "A Power widget for the Awesome Window Manager",
+   detailed = [[
+    Monitor your power devices in Awesome with UPower and DBus.
+    ]],
+   homepage = "https://github.com/stefano-m/awesome-power_widget",
+   license = "GPL v3"
+}
+supported_platforms = {
+   "linux"
+}
+dependencies = {
+   "lua >= 5.1",
+   "upower_dbus >= 0.3.0, < 0.4"
+}
+build = {
+   type = "builtin",
+   modules = {
+      power_widget = "power_widget.lua"
+   }
+}