Add bin/console script

The script starts an IRB console session in the context of the gem.
This is useful during development to test something quickly.

No database operations will work out of the box, though. This is a
future topic.

bin/console

@@ -0,0 +1,7 @@
+#!/usr/bin/env ruby
+
+require "irb"
+require "bundler/setup"
+require_relative "../lib/monarch_migrate"
+
+binding.irb