Inline assets in the CSS using Sprockets

This proc changes `url()` calls in *.css
e.g.) background-image: url("DataTables-1.10.20/images/sort_both.png");
into base64 encoded inline image via `rake assets:compile`.

Note that all images referenced in CSS will be loaded this way, whether
or not the user enables SIMPLECOV_INLINE_ASSETS option.

This idea and code is taken from
#113 by @AMHOL

Co-authored-by: AMHOL <[email protected]>

Author: amatsuda

Rakefile

@@ -36,6 +36,10 @@ namespace :assets do
     require "sprockets"
     require "sprockets/sass_processor"
 
+    Sprockets.register_processor("text/css") do |input|
+      {data: input[:data].gsub(/(?<!-|_)url\(['"]?(.+?)['"]?\)/) { "asset-data-url(\"#{Regexp.last_match(1)}\")" }}
+    end
+
     Sprockets.register_processor "text/css", Sprockets::ScssProcessor
 
     assets = Sprockets::Environment.new do |env|