title: How to use a custom Transformer for Gridsome?
body: <p>I have a proprietary file format that I'd like to use in my Gridsome website, with each of these files generating a new page. As I understand it, that's exactly what you can use a Transformer for. However, no plugin exists for a Transformer of the file type I'm using. Is it possible to create your own Transformer for private use?</p>
  <p>I've first tried simply adding the <code>source-filesystem</code> plugin, but that gives me the error: <code>No transformer for 'application/myformat' is installed.</code></p>
  <pre><code>plugins: [
        {
            use: &quot;@gridsome/source-filesystem&quot;,
            options: {
                path: &quot;files/**/*.myformat&quot;,
                typeName: &quot;File&quot;
            }
        },
    ]
  </code></pre>
  <p>I've found no documentation to do something similar, which surprised me, since it seems like it must be a fairly common use-case. Anyone know a way to do this?</p>