title: Add onclick event to nodes in Sigma.js
body: <p>I have a very simple example in sigma.js that reads a gexf-file with some additional data. </p>
  
  <pre><code>// Instanciate sigma.js and customize rendering :
  var sigInst = sigma.init(document.getElementById('graph-container')).drawingProperties({
      defaultLabelColor: '#fff',
      defaultLabelSize: 14,
      defaultLabelBGColor: '#fff',
      defaultLabelHoverColor: '#000',
      labelThreshold: 6,
      defaultEdgeType: 'straight'
  }).graphProperties({
      minNodeSize: 0.5,
      maxNodeSize: 5,
      minEdgeSize: 1,
      maxEdgeSize: 1
  }).mouseProperties({
      maxRatio: 4
  });
  
  // Parse a GEXF encoded file to fill the graph
  // (requires "sigma.parseGexf.js" to be included)
  sigInst.parseGexf(gexfFile);
  </code></pre>
  
  <p>This is actually just taken from a tutorial. Now I'd like to add an on-click event to all nodes. Can anyone tell me a proper way to do this? </p>