Add rake:debug task to quickly build the CSS

Previously to ensure that changes have been working it was required to
build the gem and then push it to a sample application (e.g. Rails &
Compass).

The rake:debug task uses plain Sass - as we do for testing, and creates
the file 'debug.css' without having to rebuild the Gem and fire up an
application.

.gitignore

@@ -1,2 +1,3 @@
 *.gem
-.sass-cache
+.sass-cache
+debug.css

Rakefile

@@ -5,4 +5,14 @@ Rake::TestTask.new do |t|
   t.verbose = true
 end
 
+desc 'Dumps output to a CSS file for testing'
+task :debug do
+  require 'sass'
+  require './lib/bootstrap-sass/compass_functions'
+  require './lib/bootstrap-sass/rails_functions'
+  path = './vendor/assets/stylesheets'
+  engine = Sass::Engine.for_file("#{path}/_bootstrap.scss", syntax: :scss, load_paths: [path])
+  File.open('./debug.css', 'w') { |f| f.write(engine.render) }
+end
+
 task default: :test