Add screenshot generator for themes (and screenshots themselves)

config.rb

@@ -1,3 +1,4 @@
+require 'lib/screenshot_generator'
 ###
 # Compass
 ###
@@ -55,6 +56,8 @@ set :images_dir, 'images'
 
 # Build-specific configuration
 configure :build do
+  activate :screenshot_generator
+
   # For example, change the Compass output style for deployment
   # activate :minify_css
 
@@ -78,4 +81,4 @@ activate :deploy do |deploy|
   # deploy.branch   = 'custom-branch' # default: gh-pages
   # deploy.strategy = :submodule      # commit strategy: can be :force_push or :submodule, default: :force_push
   # deploy.commit_message = 'custom-message'      # commit message (can be empty), default: Automated commit at `timestamp` by middleman-deploy `version`
-end
+end