title: Where does the sass "font-url" method get defined?
body: <p><a href="https://github.com/thoughtbot/bourbon" rel="noreferrer">bourbon</a> uses <code>font-url</code> <a href="https://github.com/thoughtbot/bourbon/blob/master/app/assets/stylesheets/css3/_font-face.scss" rel="noreferrer">here</a>.</p>
  
  <p>Rails has the method <code>font_url</code> which I'm fairly certain is what is being invoked. However, I can't find where the connection between these two things is made. I have explored the codebases of bourbon, sass, sass-rais, and rails.</p>
  
  <p>Where is <code>font-url</code> defined, and/or the connection between it and rails's <code>font_url</code> made?</p>
  
  <p><strong>update</strong></p>
  
  <p>Clarification: my ultimate goal is to define my own helpers in rubyland which are siblings to font_url.</p>