title: Extend spdlog for custom type
body: <p>Is there a way to extend <code>spdlog</code> to support a custom struct as an item when formatting using <code>{}</code>?</p>
  
  <p>So when I have a</p>
  
  <pre><code>struct p {
      int x;
      int y;
      int z;
  };
  
  p my_p;
  </code></pre>
  
  <p>I want to do</p>
  
  <pre><code>spdlog::info("p = {}", my_p);
  // after registering some kind of formatter object for {p}
  </code></pre>
  
  <p>instead of</p>
  
  <pre><code>spdlog::info("p = (x={}, y={}, z={})", my_p.x, my_p.y, my_p.z);
  </code></pre>