title: Change site.url to localhost during jekyll local development
body: <p>My jekyll blog template have links to resources and pages like so:</p>
  
  <pre><code>{{ site.url }}/my-page.html
  </code></pre>
  
  <p>This works well in deployment, but when I run <code>jekyll serve</code> in development, all of the links point to the live page instead of the development page.</p>
  
  <pre><code>my-site-url/my-page.html
  
  # But I want this in development
  localhost:4000/my-page.html
  </code></pre>
  
  <p>Is there a way to make jekyll use a different <code>{{ site.url }}</code> in development?</p>