title: Custom zoom buttons for Leaflet don't work
body: <p>I would want to use custom zoom buttons outside the map to save space for mobile screens. I tried this but the buttons don't work:</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;button type='button' id='zoomIn'&gt;Zoom in&lt;/button&gt;
  &lt;button type='button' id='zoomOut'&gt;Zoom out&lt;/button&gt;
  </code></pre>
  
  <p>Javascript:</p>
  
  <pre><code>function zoomIn() {
  map.setZoom(map.getZoom() + 1);
  }
  
  function zoomOut() {
  map.setZoom(map.getZoom() - 1);
  }
  
  var zoomIn = document.getElementById("zoomIn");
  zoomIn.setAttribute("onclick","javascript: zoomIn();");
  
  var zoomOut = document.getElementById("zoomOut");
  zoomOut.setAttribute("onclick","javascript: zoomOut();");
  </code></pre>