Cleanup the thumbnail generator script

lib/screenshot_generator.rb

@@ -5,12 +5,14 @@ class ScreenshotGenerator < ::Middleman::Extension
 
   def initialize(app, options_hash={}, &block)
     super
-    app.before_build do
+    app.before_build do |builder|
+      builder.say "==> Begin - Theme thumbnail generator", :green
       Screenshot.prepare
 
       themes.each do |theme|
-        Screenshot.new(theme).generate_theme_image
+        Screenshot.new(theme).generate_theme_image(builder)
       end
+      builder.say "==> End   - Theme thumbnail generator", :green
     end
   end
 end