Gem markdown_helper Adds Feature: Run Irb

I've added a feature to markdown_helper: the ability to execute markdown-embedded Ruby snippets in the Ruby interactive shell, irb.

• Use case
• Project documentation
• Gem

Reminder: markdown_helper is available via the command line, so you can use it without writing Ruby code:

• File inclusion: markdown_helper include [options] includer.md included.md
• Run irb: markdown_helper run_irb [options] template.md markdown.md

And, as before, also available via its Ruby API.