title: conditionally render siteUrl property in Gatsby-config based on environment
body: <p>I would like to set up a simple expression in my <code>gatsby-config.js</code> that determines whether I am working locally or in production so that I can set the <code>siteUrl</code> property appropriately.</p>
  <p>My gatsby-config <em>(with psuedo code)</em>:</p>
  <pre><code>module.exports = {
      siteMetadata: {
          title: &quot;My title&quot;,
          description: &quot;My description...&quot;,
          siteUrl: (process.env=DEVELOPMENT) ? &quot;/&quot; : &quot;https://example.com /* something here */
    }, {
       ... 
    }
  </code></pre>
  <p>Is this possible with simple JS, string literals, etc.. ?</p>