title: chart.js 2.0 add new property to dataset
body: <p>i need to add one property to dataset/label to identify it on click. 
  I've tried something like this: </p>
  
  <pre><code>Chart.controllers.customBar = Chart.controllers.bar.extend({
          code: null
      });
  </code></pre>
  
  <p>and it almost works, because i can see new property "code", data is associated with it, chart background is visible but empty, i mean no data from datasets visible.</p>
  
  <p>This is my initialization:</p>
  
  <pre><code>var ctx = document.getElementById("roads-chart").getContext("2d");
      window.roadsBar = new Chart(ctx, {
          type: 'bar',
          data: roadsChartData,
          options: {
              title: {
                  display: true,
                  text: "Stan dróg"
              },
              tooltips: {
                  mode: 'label'
              },
              responsive: true,
              scales: {
                  xAxes: [{
                          stacked: true,
                      }],
                  yAxes: [{
                          stacked: true
                      }]
              },
              onClick: function(e, d) {
                  console.log(d);
              }
          }
      });
  </code></pre>
  
  <p>The thing is, i don't feel Chart.js, can someone help me handle it and show an example how does extending work? :)</p>
  
  <p>Thanks</p>