beautiful: add get_image function

this commit adds awful.beautiful.get_image(imgspec) to retrieve an image
from a beautiful theme

lib/awful/beautiful.lua.in

@@ -5,6 +5,7 @@
 ---------------------------------------------------------------------------
 
 local package = package
+local image = image
 
 --- Beautiful module for awful
 module("awful.beautiful")
@@ -30,4 +31,14 @@ function get()
     return package.loaded.awful.beautiful.theme
 end
 
+--- Return an image from the theme
+-- @param imgspec name of the image key
+-- @return the image
+function get_image(imgspec)
+    if theme[imgspec] then
+        return image(theme[imgspec])
+    else
+        return nil
+    end
+end
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:encoding=utf-8:textwidth=80