Add ability to extend `rails server` command options parser

With this change it will be possible to add additional options to the `option_parser` like this:

    require 'rails/commands/server'
    module Rails
      class Server < ::Rack::Server
        class Options
          def option_parser_with_open(options)
            parser = option_parser_without_open options
            parser.on('-o', '--open', 'Open in default browser') { options[:open] = true }
            parser
          end
          alias_method_chain :option_parser, :open
        end

        def start_with_open
          start_without_open do
            `open http://localhost:3000` if options[:open]
          end
        end
        alias_method_chain :start, :open
      end
    end