Allow `Autoprefixer::Compiler` to be initialized without arguments

The way I recently managed to [integrate Autoprefixer Rails](https://github.com/rails-assets/rails-assets/blob/0951645df5ad608f2e1a6b2836cfc7b4f74dcf1c/lib/rails/assets/web.rb#L25) with Sinatra app’s Sprockets requires `Autoprefixer::Compiler` to be initialized. I wish I could have just typed `Autoprefixer::Compiler.new` instead of ugly `Autoprefixer::Compiler.new(nil)`.