Add Sass processing of docs css files

This was removed in bcc3dd84d0 but is
still needed.

Gemfile

@@ -4,6 +4,7 @@ gemspec
 group :docs do
   gem "yard", "~> 0.8.0"
   gem "kramdown"
+  gem "sass"
 end
 
 platform :mri do

Rakefile

@@ -54,6 +54,16 @@ begin
   end
 
   namespace :doc do
+
+    task :sass do
+      require 'sass'
+      Dir["yard/default/**/*.sass"].each do |sass|
+        File.open(sass.gsub(/sass$/, 'css'), 'w') do |f|
+          f.write(Sass::Engine.new(File.read(sass)).render)
+        end
+      end
+    end
+
     desc "List all undocumented methods and classes."
     task :undocumented do
       command = 'yard --list --query '
@@ -64,7 +74,7 @@ begin
   end
 
   desc "Generate documentation"
-  task(:doc) {sh "yard"}
+  task(:doc => 'doc:sass') {sh "yard"}
 
   desc "Generate documentation incrementally"
   task(:redoc) {sh "yard -c"}