title: Plotly.js modebar, download as png, give png a name
body: <p>I have a Plotly on my webpage and you can download it as a png by clicking the picture icon in the modebar. However when I click it, it downloads it as a png with the name new-plot, how can I give it a custom name?</p>
  
  <p>My current code (var data is just data, so left it out) :</p>
  
  <pre><code>var layout = {
      showlegend: true,
      legend: {
          x: 0,
          y: 1
      },
      xaxis: {
          title: 'Date',
          titlefont: {
              family: 'Courier New, monospace',
              size: 18,
              color: '#7f7f7f'
          }
      },
      yaxis: {
          title: 'Sales',
          titlefont: {
              family: 'Courier New, monospace',
              size: 18,
              color: '#7f7f7f'
          }
      }
  };
  
  var options = {
      scrollZoom: true,
      showLink: false,
      modeBarButtonsToRemove: ['zoom2d', 'pan', 'pan2d', 'sendDataToCloud', 'hoverClosestCartesian', 'autoScale2d'],
      displaylogo: false,
      displayModeBar: true,
  };
  
  Plotly.newPlot('tester', data, layout, options);
  </code></pre>