chore(bin): Add bin/console for better DX

I wanted to try RDoc methods and noticed there's no `bin` dir.
This `bin/console` loads `doc/rdoc/markup_reference` for easy
testing and debugging.

Author: okuramasafumi

bin/console

@@ -0,0 +1,9 @@
+#!/usr/bin/env ruby
+
+require 'bundler/setup'
+require 'rdoc'
+
+require_relative '../doc/rdoc/markup_reference'
+
+require 'irb'
+IRB.start(__FILE__)