title: Is there a way to set set config.action_controller.asset_host in development
body: <p>In my production.rb I set my asset_host to CloudFront like so:</p>
  
  <pre><code>config.action_controller.asset_host = 'http://xxxxxxxx.cloudfront.net'
  </code></pre>
  
  <p>Now I'm finding that in some circumstances (specifically, outputting JavaScript to be embedded into another site) I need to set the asset_host in the development environment too, the default null won't cut it. Ideally I want to set: </p>
  
  <pre><code>config.action_controller.asset_host = 'http://localhost:3000'
  </code></pre>
  
  <p>but this port can't be guaranteed, and I'm reluctant to hard-code it. Is there a way to set asset_host to the current domain and port?</p>
  
  <p>Thanks!</p>