title: Ruby post title to slug
body: <p>How should I convert a post title to a slug in Ruby?</p>
  
  <p>The title can have any characters, but I only want the slug to allow <code>[a-z0-9-_]</code> (Should it allow any other characters?).</p>
  
  <p>So basically:</p>
  
  <ul>
  <li>downcase all letters</li>
  <li>convert spaces to hyphens</li>
  <li>delete extraneous characters</li>
  </ul>