Add beforeopen event

[jonataswalker]

If you want to disable this plugin under certain circumstances, listen to beforeopen:

contextmenu.on('beforeopen', function(evt){
  var feature = map.forEachFeatureAtPixel(evt.pixel, function(ft, l){
    return ft;
  });

  if (feature) { // open only on features
    contextmenu.disable();
  } else {
    contextmenu.enable();
  }
});