Adding Excel xlsx support and prefixed images

* New sample!
* New API function!
* Ignoring _img in samples

lib/squib/graphics/save_images.rb

@@ -1,8 +1,8 @@
 module Squib
   class Card
 
-    def save_png(i, dir: '_img')
-      cairo_context.target.write_to_png("#{dir}/img_#{i}.png")
+    def save_png(i, dir, prefix)
+      cairo_context.target.write_to_png("#{dir}/#{prefix}#{i}.png")
     end
 
   end